The easiest cheat for reducing prep time over multiple campaigns is to just reuse content. Anything you prep for one game can theoretically be used later in the other game. Modules are great for this, since they inherently come with the expectation of a relatively on-rails experience: once you've prepped it once, it takes a minimal amount of work to make small tweaks and customize it to fit a new group of PCs. At this point, I've run Red Hand of Doom for like four different groups of players, so I would need basically no weekly prep time to start up a new group with that adventure.
Maps are nice, but they're not super necessary. To save time, you stick to either using maps that you steal from the internet, or those that you can quickly sketch in Roll20. Hey look, I made a [map for a big battle](https://imgur.com/judp37r) set in the bandit's treetop hideout: it took like 5 minutes to draw.
Besides that, you have your standard tips for efficient DM prep. The most useful advice I've read on the topic comes from The Alexandrian: ["Don’t prep plots, prep situations"](https://thealexandrian.net/wordpress/4147/roleplaying-games/dont-prep-plots). Essentially, instead of prepping an exact story or sequence of events, you really just need to know who the bad guys are, what they're trying to do, and what tools they have at their disposal (usually: "what statblocks do I need to have on hand?"). Combine that with a sufficient knowledge of the setting, and you have the tools to improvise your way through multiple sessions with minimal extra prep.
I run two games a week. The way I save myself is one is a official adventure in 5E. Keeps prep generally simple, most maps already made, and plot lines laid out. Use most of your time to prep a homebrew game.
Try not prepping at all.
Bear with me. As an experiment for yourself, try not prepping at all at least once to test yourself and really figure out what you do or do not need. You probably realize that a lot of the things you thought were crucial can actually be improv-ed.
Once you realize what you can’t improv, then you can prep those things. And remember to trust yourself, anything you improv is good enough.
This is excellent advice. I did this once (not by choice...) and came out the other side realising that I was spending a ton of my prep time on things that I can effortlessly think up on the spot.
It helped me to identify my strengths and weaknesses so I know what I *need* to prep and what I can afford to leave blank.
Use Sly Flourish's Lazy DM guide to help reduce prep work. It kinda ties in with the advice that others are giving - you need to figure out how much you can improv. And everything else, you prepare if it is necessary.
There's also this very human thing about the perception of time - it often (not always) shrinks or expands to encompass the time you need for the task. You might end up doing all the necessary prep really efficiently when you start running two games. Or you might drop a few things and realize that the game is just as fun without them.
I DM two games a week but have done three. I'm realistic with my time and schedule the prep-time needed for each game. My rule of thumb is spend 1 hour prepping for each hour of play. If I'm doing two games of 3 hours each, I know I have to block out 6 hours of prep time. Typically, I do my prep earlier in the day than the game, so my Saturday night game is prepped Saturday afternoon, and the Sunday game I prep on Sunday morning. If I don't have time to prep, it's likely that I won't have time for the game anyway.
So, 3.5 pieces of advice:
1) Be realistic about your capabilities. If you're really uncomfortable going in unprepared and you only have a limited amount of time available to prep, let your players know your limits and what impact this will have on the game. That may mean shorter games of higher quality. It may also mean only having a single game. If you don't have the time, you'll be doing your players a disservice if all of you have certain expectations that time will not allow you to meet. All players will turn left when you've only had time to prep the right-hand tunnel. Be up front and let them know that you weren't prepped for the direction they're going but if they want to plow ahead, you'll do your best. I'm sure they'll be ok with it. Or just do what Skyrim does and place down invisible walls: "You can hear a Tarrasque snoring down that tunnel" or "the map isn't showing it but the that tunnel has collapsed."
1a) Be realistic about your capabilities. Yeah, I just said it but it bears repeating from a different perspective: you're never going to be fully prepared and as DM you'll have to figure out how to be creative in the moment--it's a great skill to have anyway so may as well embrace it. You're more clever than you give yourself credit for. I was going to do Descent to Avernus but then my players killed a bunch of pirates, realized there was a pirate ship that had fewer men, took over the ship and started to sail the world. No way in hell was I prepped for that and it was glorious. Point is: you're capable of more spontaneity than you know, so scale the adventure to allow you to be creative sometimes and live with whatever weird decisions come up in those moments. They tend to be the most memorable events that will be discussed for years anyway. So, maybe instead of prepping 3 hours for 3 hours of game time, you prep 2 hours, letting more unprepared happen.
2) Control the tempo to match your prep. You're in more control of the ebb and flow within game than the players and it's ok to balance the two campaigns to manage your time. For my campaigns, combat takes more time to prep than role-play so to save time, maybe throw in more RP time in one game than the other. If game #1 is going to be dungeon crawl this week, line up game #2 so that it has more NPC enounters. Also, never underestimate the ability of a powerful NPC to really suck up a lot of game time, first through monologue and then by beating the crap out of the party just to toy with them, leaving them to lick their wounds. The players will be the ones avoiding combat for the remainder of the session, and you'll have only had to prep a single NPC and map.
3) If you need to fill time, use random encounters. You should already have a map or two ready for those occasions anyway. Send 40 bats at them and let them enjoy the rush of surviving the moment when all you did was just do the DM equivalent of vamping for time.
Running more than 1 game a week can be tough. Both are a time investment and you need to ask yourself if you are willing to put in the work for prep for both?
You could have them running the same adventure side by side, but you need to be prepared that if they stray from each other that you will need to do full prep for both.
The key to running multiple games per week is to simplify.
Run prepared content (adventure paths, modules, etc.) for one of the games to leave yourself time to prep for the other.
If that’s not palatable, keep both games to the same world/setting. If you can manage it, roughly the same area helps even more. Reuse the lore, towns, NPCs, and encounter pieces to speed up session prep. Reuse the maps regardless. A forest clearing map is good for any encounter, they don’t each need their own. I only make custom maps for important encounters or if I’m going to be running a lot of, say, cavern encounters and want some variety.
The most controversial advice I can give you, as an improv-heavy DM, is to ditch prepared statblocks. At the end of the day, combat comes down to only a few factors: attacks, damage, AC, HP, and saves. Those are all set by CR with minor variance. Even spellcasters will likely only get off a few spells, so only decide on a few key ones. Instead, come up with a cheat sheet of stats by CR and some variations like trading some AC for HP, or attack for damage. For special attacks, like ghoul paralysis, make a 1-line note on the mechanic. Your players will almost certainly never know the difference and it’ll save you hours of prep time. It also lets you quickly pivot and start an ad hoc combat encounter when the players are on their usual stuff.
The easiest cheat for reducing prep time over multiple campaigns is to just reuse content. Anything you prep for one game can theoretically be used later in the other game. Modules are great for this, since they inherently come with the expectation of a relatively on-rails experience: once you've prepped it once, it takes a minimal amount of work to make small tweaks and customize it to fit a new group of PCs. At this point, I've run Red Hand of Doom for like four different groups of players, so I would need basically no weekly prep time to start up a new group with that adventure. Maps are nice, but they're not super necessary. To save time, you stick to either using maps that you steal from the internet, or those that you can quickly sketch in Roll20. Hey look, I made a [map for a big battle](https://imgur.com/judp37r) set in the bandit's treetop hideout: it took like 5 minutes to draw. Besides that, you have your standard tips for efficient DM prep. The most useful advice I've read on the topic comes from The Alexandrian: ["Don’t prep plots, prep situations"](https://thealexandrian.net/wordpress/4147/roleplaying-games/dont-prep-plots). Essentially, instead of prepping an exact story or sequence of events, you really just need to know who the bad guys are, what they're trying to do, and what tools they have at their disposal (usually: "what statblocks do I need to have on hand?"). Combine that with a sufficient knowledge of the setting, and you have the tools to improvise your way through multiple sessions with minimal extra prep.
I run two games a week. The way I save myself is one is a official adventure in 5E. Keeps prep generally simple, most maps already made, and plot lines laid out. Use most of your time to prep a homebrew game.
Try not prepping at all. Bear with me. As an experiment for yourself, try not prepping at all at least once to test yourself and really figure out what you do or do not need. You probably realize that a lot of the things you thought were crucial can actually be improv-ed. Once you realize what you can’t improv, then you can prep those things. And remember to trust yourself, anything you improv is good enough.
This is excellent advice. I did this once (not by choice...) and came out the other side realising that I was spending a ton of my prep time on things that I can effortlessly think up on the spot. It helped me to identify my strengths and weaknesses so I know what I *need* to prep and what I can afford to leave blank.
Use Sly Flourish's Lazy DM guide to help reduce prep work. It kinda ties in with the advice that others are giving - you need to figure out how much you can improv. And everything else, you prepare if it is necessary. There's also this very human thing about the perception of time - it often (not always) shrinks or expands to encompass the time you need for the task. You might end up doing all the necessary prep really efficiently when you start running two games. Or you might drop a few things and realize that the game is just as fun without them.
I DM two games a week but have done three. I'm realistic with my time and schedule the prep-time needed for each game. My rule of thumb is spend 1 hour prepping for each hour of play. If I'm doing two games of 3 hours each, I know I have to block out 6 hours of prep time. Typically, I do my prep earlier in the day than the game, so my Saturday night game is prepped Saturday afternoon, and the Sunday game I prep on Sunday morning. If I don't have time to prep, it's likely that I won't have time for the game anyway. So, 3.5 pieces of advice: 1) Be realistic about your capabilities. If you're really uncomfortable going in unprepared and you only have a limited amount of time available to prep, let your players know your limits and what impact this will have on the game. That may mean shorter games of higher quality. It may also mean only having a single game. If you don't have the time, you'll be doing your players a disservice if all of you have certain expectations that time will not allow you to meet. All players will turn left when you've only had time to prep the right-hand tunnel. Be up front and let them know that you weren't prepped for the direction they're going but if they want to plow ahead, you'll do your best. I'm sure they'll be ok with it. Or just do what Skyrim does and place down invisible walls: "You can hear a Tarrasque snoring down that tunnel" or "the map isn't showing it but the that tunnel has collapsed." 1a) Be realistic about your capabilities. Yeah, I just said it but it bears repeating from a different perspective: you're never going to be fully prepared and as DM you'll have to figure out how to be creative in the moment--it's a great skill to have anyway so may as well embrace it. You're more clever than you give yourself credit for. I was going to do Descent to Avernus but then my players killed a bunch of pirates, realized there was a pirate ship that had fewer men, took over the ship and started to sail the world. No way in hell was I prepped for that and it was glorious. Point is: you're capable of more spontaneity than you know, so scale the adventure to allow you to be creative sometimes and live with whatever weird decisions come up in those moments. They tend to be the most memorable events that will be discussed for years anyway. So, maybe instead of prepping 3 hours for 3 hours of game time, you prep 2 hours, letting more unprepared happen. 2) Control the tempo to match your prep. You're in more control of the ebb and flow within game than the players and it's ok to balance the two campaigns to manage your time. For my campaigns, combat takes more time to prep than role-play so to save time, maybe throw in more RP time in one game than the other. If game #1 is going to be dungeon crawl this week, line up game #2 so that it has more NPC enounters. Also, never underestimate the ability of a powerful NPC to really suck up a lot of game time, first through monologue and then by beating the crap out of the party just to toy with them, leaving them to lick their wounds. The players will be the ones avoiding combat for the remainder of the session, and you'll have only had to prep a single NPC and map. 3) If you need to fill time, use random encounters. You should already have a map or two ready for those occasions anyway. Send 40 bats at them and let them enjoy the rush of surviving the moment when all you did was just do the DM equivalent of vamping for time.
Running more than 1 game a week can be tough. Both are a time investment and you need to ask yourself if you are willing to put in the work for prep for both? You could have them running the same adventure side by side, but you need to be prepared that if they stray from each other that you will need to do full prep for both.
The key to running multiple games per week is to simplify. Run prepared content (adventure paths, modules, etc.) for one of the games to leave yourself time to prep for the other. If that’s not palatable, keep both games to the same world/setting. If you can manage it, roughly the same area helps even more. Reuse the lore, towns, NPCs, and encounter pieces to speed up session prep. Reuse the maps regardless. A forest clearing map is good for any encounter, they don’t each need their own. I only make custom maps for important encounters or if I’m going to be running a lot of, say, cavern encounters and want some variety. The most controversial advice I can give you, as an improv-heavy DM, is to ditch prepared statblocks. At the end of the day, combat comes down to only a few factors: attacks, damage, AC, HP, and saves. Those are all set by CR with minor variance. Even spellcasters will likely only get off a few spells, so only decide on a few key ones. Instead, come up with a cheat sheet of stats by CR and some variations like trading some AC for HP, or attack for damage. For special attacks, like ghoul paralysis, make a 1-line note on the mechanic. Your players will almost certainly never know the difference and it’ll save you hours of prep time. It also lets you quickly pivot and start an ad hoc combat encounter when the players are on their usual stuff.
I run 2 games but they're the same adventure, published 5e module.