Some window managers are offering an option, to change the user (plasma = KDE does it, for example.). It is also possible, to start a new X-session and login with another user. Doing so, you have 2 X-session open.
The option is also, to change the user, or, add another user with a new session. Sometimes, I need to start an application as root in X (for example, if I want to edit a configuration file, only rott is allowed to edit) ands I want to use a graphical writer, like kwrite (just an example). Instead of logging out and relogon into the window manager as root, I am starting a shell., and enter "su -p" (this meany, preserve my environment). Thus I can start any graphical application from the shell (starting kwrite with the command) and it will start with root permissions. In earlier times, there was "sux" a wrapper for su, but this is no more available. I never found an official documentation about "su -p", just found it myself, but I read, "su -" shall do the same. It does not. Maybe your question aimed to this scenario, if so, there is my solution. Have fun! Best Hans
