I'm always happy if somebody wants to chime in here to correct me, add to what I have to say, but here's a summary of what I believe I know about this subject.
TLDR: As far as I know, using one computer with a single, powerful GPU in combination with display-extending hardware tends to be better than using a computer with multiple discrete GPUs (and also better than trying to synch multiple computers if you care about your outputs being frame-synched).
@armando said:
having multiple graphics cards is supported on Macs right now?
I should have been more specific. It actually says, "Computers with multiple discrete GPUs that have not been properly configured" and "Computers with external graphics cards (eGPUs)".
So yes, actually, Apple and Windows computers with multiple internal graphics cards are supported but only when configured properly. To the best of my knowledge though, there are very few recent Apple computers that support the use of multiple internal graphics cards. The only ones I know off the top of my head are:
The old, "cheese grater" Mac Pro towers (which at this point are more than a decade old).The 2013 Mac Pro (commonly called "trash cans" because of the shape), which are also over a decade old and also have the issue that few, if any, programs can actually make use of the dual graphics card setup. One of these has been my main show computer since 2015 and, while I've needed to get it repaired a few times, I found that the number of outputs it offers is usually enough, and when it isn't, one can use display-extending hardware or BlackSyphon + direct output to Blackmagic hardware to get more outputs.The newer "cheese grater" Mac Pro towers (which are so expensive I've never even considered trying one out).
Frankly, I believe there are better options. For Windows it's optimal to go with a single, powerful graphics card with as many outputs as possible then use display extenders like the Matrox TripleHead2Go DP Edition, Matrox QuadHead2Go, or Datapath fx4 (which has the added benefit of being daisychain-able) as this is (as far as I understand) is more efficient since textures don't need to be passed between multiple graphics cards. I've heard from other people, (who are more experienced with Windows than I am), that if you can figure out the proper configuration, using multiple discrete internal GPUs isn't an issue, but personally I still prefer the external display-extending hardware because it can easily be used with different computers for different projects, which I find more flexible than building a computer with multiple, discrete internal GPUs.
For macOS, I've heard that the Mac Studios are quite powerful and, again, you can use display extending hardware to get more outputs if needed. These display extenders can also make it easier to keep your outputs frame-synched (important when using multiple beamers or displays to create a larger, composite image) as the computer sees each display extender as a single, large display. So, with the example of a Datapath fx4 or Matrox QH2G, instead of sending four separate videos to four separate displays, your computer sends different parts of the same high-resolution video to something it sees as single display, resulting in all four displays connected to the display extender being frame-synched.
So overall, I personally think there's better, more efficient, options than trying to work with a machine that has multiple graphics cards. Again, though, I'm happy to be corrected if anyone knows more about this than me. I'm always open to learning, especially when it results in finding out about a better way to do something.