You want two or more users to be able to run on seperate X Window System sessions on the same system. Obviously not interacting with the system at the same time if both sessions are running on console, but not havin to bail out of a session to allow the other user access.

Do a console login as the desired userid. Launch an X Window System session to an unused local display. Assuming your first session runs to :0, try:

startx -- :1 1>.startx.log 2>&1 &

For gdm, add the following lines to the file ....