I do have a Multiseat setup with 2 seats and 2 (different) nvidia-cards using nouveau. It was nessecary to use 2 cards because i could not get it working with one card to have 2 seats 
After some work this works fine since a few years.
Now i may need to replace the older card. Amd-cards are much cheaper then nvidia-cards.
- But is ist possible to use nvidia-card for seat0
- and amd-card for seat1
Does anyone have such a setup running ?
P.S. I am no gamer, but a developer 
, and i do know that the risk will stay at my side.
I have seen multiseat - posts with 2 AMD-Card and posts with 2 Nvidia-cards, but nothing mixed ;-(