import the X-display of one local ubuntu user onto another user's desktop

Inspired from this topic I dare ask this probably impossible question:

I have 4 different users on my Ubuntu-MATE 21.04 machine. Sometimes it would be nice to see user2 on the display of user1, either in a Window (some sort of exported X-Window, rlogin,...) or even better, one of those 4 Workspace Panels:

So like Panel 1-3 for user 1, Panel 4 for user 2. (I am aware that I can do sudo - …, rlogin, ssh on the command line to run certain commands as another user, but now+then I would truly need that other use with the actual GUI...)

If there is a solution, perhaps it is in the world of export DISPLAY=:0.0 NO_AT_BRIDGE=1 or in this answer. Perhaps exporting is easy and I just need a rather dumb X-Display client? As you can tell, I have very little clue around Displays, XServer and Exports.