I want to setup a dual head system with a monitor and a TV. I want to have two totally separate X window desktops (different users logged in) displayed, but share one keyboard and mouse between them. Can this be done?

I almost have it working using default gdm for graphical login and a custom, but typical, xinerama-like setup in xorg.conf (not twinview! I just use different monitor, screen, and device sections for the monitor and TV, and combine them in a single ServerLayout). There are two different desktops, and the $DISPLAY variables are :0.0 (monitor) and :0.1 (TV). Specifically, the screen is not cloned nor is it a single panoramic desktop. The screens share keyboard and mouse by moving the mouse off the right edge of the first screen onto the second screen to shift focus. This is really close to what I want, but I do want to be able to login as different users on the different screens.

If I use gdmflexiserver, crtl-alt-f8, or similar commands (see this useful thread) on one screen, both screens are simultaneously provided the gdm login screen. Ideally, only the active screen would ask for a new login.

One potential route is Xnest or gdmXnest, but they only open up new X servers for the current user. Can you have it come up with a gdm login screen?