Multiple monitors on separate GPUs

I’m either doing something wrong or what I want is simply impossible.

There are two GPU’s in my system, each has a monitor connected to it and I simply want to use them as two displays as you would if they were both connected to either one of the GPU’s.

However, in xorg.conf I need to setup two screens and this second screen never shows up as usable if I goto “Display Configuration” it does however work for the login prompt but after login it just stays at a blank Xscreen.

Sure, I could get it to “work” with Xinerama as one big screen. But then I have just one screen, which makes applications maximize using the entire width of the screen and gaming with Steam/Wine impossible.

How do I set it up to drive two screens using two GPU’s?

It depends on the type of gpus. With a mixed setup, you’re left with Xinerama and need a WM that supports Xinerama or the randr 1.5 monitor objects.
If the gpus are the same, you could try BaseMosaic but that is a Quadro feature, rarely working with GeForce type cards. Overall, it doesn’t really make sense to use two graphics cards in that way. gpu performance doesn’t add up.