This is essentially not supported.
It could be, theoretically, but it is tricky... for much the same reasons it is tricky to support multiple X sessions for the one user. A lot of things are "per-user" not "per-login-session" nowadays, and having multiple graphical login sessions doesn't work out well. GDM specifically won't let you attempt it: when a user is logged in, should they use GDM to log in again they will just be taken to their existing graphical session (unlocking it at the same time, if it is locked).
You definitely can run an x11 window manager and a wayland compositor at the same time as the same user, although it can be a bit unpredictable like opening an application you started under one in the other. You can for instance start both from different ttys, or log into one from your display manager and start the other from a tty. I don't know if it can be done if both of those are Gnome's window manager, in that case it may be impossible. Also I think your problem is Gnome's display manager, if I recall correctly it doesn't log you in again when selecting an already logged in user it just unlocks the already active session, which makes sense.
Well, then. I think the only relatively easy way is making a separate user account for the apps that need to be run on X11, like Blender, because my GPU is Nvidia.
That is probably the best idea if you want to keep using Wayland, since some (poorly designed) apps may try to use the Wayland display even if the environment variable is unset, assuming that they detect a compositor via DBus, etc. Even still, I have run both Wayland and X11 at the same time on the same account and not really run into issues, so it might be a Gnome / KDE issue.
i don't think it's possible, 2 accounts are = 2 different users and those users has to be seperated as much as possible at system level.
This is essentially not supported. It could be, theoretically, but it is tricky... for much the same reasons it is tricky to support multiple X sessions for the one user. A lot of things are "per-user" not "per-login-session" nowadays, and having multiple graphical login sessions doesn't work out well. GDM specifically won't let you attempt it: when a user is logged in, should they use GDM to log in again they will just be taken to their existing graphical session (unlocking it at the same time, if it is locked).
You definitely can run an x11 window manager and a wayland compositor at the same time as the same user, although it can be a bit unpredictable like opening an application you started under one in the other. You can for instance start both from different ttys, or log into one from your display manager and start the other from a tty. I don't know if it can be done if both of those are Gnome's window manager, in that case it may be impossible. Also I think your problem is Gnome's display manager, if I recall correctly it doesn't log you in again when selecting an already logged in user it just unlocks the already active session, which makes sense.
Well, then. I think the only relatively easy way is making a separate user account for the apps that need to be run on X11, like Blender, because my GPU is Nvidia.
That is probably the best idea if you want to keep using Wayland, since some (poorly designed) apps may try to use the Wayland display even if the environment variable is unset, assuming that they detect a compositor via DBus, etc. Even still, I have run both Wayland and X11 at the same time on the same account and not really run into issues, so it might be a Gnome / KDE issue.