Totally possible. It’ll work best with Wayland thanks to nested compositor support, whereas on Xorg you’d need to use Xephyr which doesn’t do hardware acceleration.
<span style="color:#323232;"># Give the other user access to your Wayland socket
</span><span style="color:#323232;">setfacl -m u:otheruser:rx $XDG_RUNTIME_DIR
</span><span style="color:#323232;">setfacl -m u:otheruser:rwx $XDG_RUNTIME_DIR/wayland-0
</span><span style="color:#323232;">
</span><span style="color:#323232;"># Open a session as the other user (note the trailing @, it's there to login in to the local machine)
</span><span style="color:#323232;">sudo machinectl login otheruser@
</span><span style="color:#323232;">
</span><span style="color:#323232;"># Start your DE!
</span><span style="color:#323232;">WAYLAND_DISPLAY=/run/user/$(id -u yourmainuser)/wayland-0 startplasma-wayland
</span>