I've asked about multiseat before. The answer (for 10.04) involved doing it the proper way (eg through gdm, multiple server layouts). The problem was that gdm needs to be patched or reverted to 2.20 for multiseat. It's an ugly hack that, worse than anything, will hold up future updates.
As a result, I didn't do anything. I still have a spare video card. I still have the monitor, keyboard and mouse all sitting waiting to jump into action. And I still want to be able to turn that into a simple desktop.
My needs don't seem complicated. I have a second video card, a USB hub and anything connected to that USB hub that I want to be dedicated to another X server. I don't need a login screen (I'm happy hard-coding in a auto-login and I'd be happy with the user starting the X server if that's possible).
This is so simple in my head that I only need two questions:
How can I explicitly start an X server from the command line on an unused video adapter (by passing it whatever configuration I need to)?
Can I have this new X session load a desktop environment on load?
This seems like something you should be able to write in a little upstart script within 10 minutes. That would be perfect for me as then I'd have a nice start/stop control over the secondary desktop from the main desktop (that I want to leave unscathed!)
I'm thinking something as simple as this for the payload:
su -u other_user -c "startx -- localhost hardware-information"
And use .xinitrc to load openbox or something...
Why don't you use X server layouts, but not gdm? Or if you need a login manager, use something that doesn't kvetch about multiple instances (or whatever the problem with gdm is). I'm reading the blog entry that was linked to in your other question and wondering why not just ditch the gdm part of his instructions and run gnome-session or wmfs or..
The command would be: