I noticed in ximian gnome2 that gdm allows logins as another user in a regular
This facility is still not enabled in redhat. It would be useful, as there's no
easy way for newbies to login as another user without logging out the current one.
That feature uses Xnest, and isn't really supportable as Xnest lacks
some needed features. For example if any app in the window
tries to draw an icon while you're on another workspace, the Xnest will crash.
And Xnest doesn't have all the extensions and such that normal X has. Basically
Xnest isn't supported for actual usage, it's just a test program.
You can log on two users at once by using multiple virtual consoles running
real X sessions, if you poke around on the internet some there's probably a HOWTO.
If you want to also graphically log into a machine that is already logged in
graphically- jump into a virtual console:
then login, and type:
startx -- :1
To jump back to the original X session: Ctrl-Alt-F7