Hopefully this question isn't too broad - I'm mostly just looking for some 'how-to' kind of information here, but I don't have (currently) a machine configured this way.
Background: Years ago, when GPUs only supported 1-2 monitors, it was not uncommon to have 2 GPUs in a single computer to handle 3 monitors, and I had a setup like this for a long time - but it ran WindowsXP and times have changed a lot since then. Modern GPUs seem to support, on average, 4 monitors - I've tried connecting more than that to a newer GeForce and it wasn't having it (and you can, if you search the web properly, find documentation from nVidia confirming that 4 is the limit, despite the cards generally having five outputs
). I know some Radeon cards can do >4 (I have one with five outputs (that are all supposed to work 'together')), but usually that requires MST hubs (which are not cheap) or a bunch of mDP-to-something adapters (which is the case for my example Radeon).
Thus, I've tried doing 'two modern GPUs' in a machine (to run 3-4+ monitors sans adapters/etc) and had it work just as expected in Windows 7 and 8.1, but when trying it in linux (both GeForce and Radeon - using noveau, nVidia proprietary, and the xorg-radeon drivers, respectively) while both GPUs will be 'detected' getting any usable output (in X) from the second (or third, or fourth, etc) card is not something I've ever achieved.
Example: I tried this with two GeForce cards, with a GTX 660 and a GT 610 installed in the same machine (and for ease of testing, I was trying one monitor connected to each) - the GTX 660 would provide a single display in X (which was labeled screen0), and the GT 610's output was just a grey screen with the big X for a cursor, but there was no way to 'link' that (I believe it was labeled screen1) to the 660's output.
I've also tried the same configuration as above with a pair of Radeon cards (an HD 4670 and some 'lesser' model - I think it was an X1650), and got to basically the same place. I know all of my hardware works (all four GPUs that I've tried here worked just fine as the 'primary' GPU - most of them are also regularly used in other machines).
I've tried this in: Xubuntu 19.10 and 20.04, and Ubuntu 18.04 and 20.04.
Question/Query: Any help in figuring out how to set this up properly (or even some better 'search terms' to go looking after)? When I've tried searching the web for 'multiple GPUs in linux' or similar queries its usually people wanting SLI or CrossFire, or some other compute-based thing (like cryptocurrency mining or AI), not just the basic 'I plug in a second graphics card to have another monitor' approach.
