I fought with this in the past. In the end I unhappily switched to back to windows running linux VMs after years of using linux as my primary desktop OS, and i

I fought with this in the past. In the end I unhappily switched to back to windows running linux VMs after years of using linux as my primary desktop OS, and it was purely because of this failure. Don't expect multiple graphics cards to work well under linux, at all. I tried this for quite awhile and there are a number of bugs. The one that bit me the most was that when a mouse cursor cross a screen boundary that switches GPUs and there is an animated cursor running, the mouse gets all messed up with the clicks being sent to the wrong screen, click on screen 1 and the event is sent to screen 2. I also had constant issues across many version of X with the Xorg process pegging a CPU core and making the system unusable. Xinerama is all but deprecated it seems, RandR was supposed to replace the multiple graphics card support and allow multi-gpu composting but the project hasn't gotten around to it despite the fact that they've been talking about multi-gpu handling for more than 3 years. My "workaround" for a long time was using a Matrox tripleHead2Go to let me get 4 monitors onto a single nvidia card. But, this approach has inherent limitations, all displays have to be oriented the same way and have to be side by side, what I wanted was one screen in landscape, 2 in portrait on the sides and a couple extra above. At this point the best bet is hoping that wayland handles multi-gpus far better than X does.