While previously you needed to have special, more expensive motherboards running two or more video cards from the same vendor (called Crossfire/SLI by their respective vendors), DirectX 12 allows for multi-display adapter (MDA) where the software (game, mostly) can explicitly work with, as the name suggests, multiple displays (CF/SLI is now called linked display adapter or LDA)). However, to quote one of the earliest benchmarks from 2016 February:
Almost no games presently support Dx12, fewer still support this type of multi-GPU configuration, and then there still has to be some dev-side support and testing for that configuration. It's not all just about drivers and APIs; if the game's QA team hasn't run the configuration, there's a good chance more bugs present themselves.
While obviously since then more games started supporting DirectX 12, it is still true that few supports MDA and you are more likely to face bugs. If there are N video cards the QA tested then to test all possible pairs they would need to test almost N squared, which very quickly becomes unfeasible.
So: you can have two GPUs, in theory they will accelerate some games together, in practice it's a bit more dubious.
If you have no need of that and just want to hang a lot of monitors off the same PC, you can just expect that to work to an extent: applications run a specific GPU so stretching one application between monitors hanging off a different card is something unlikely to work. This might be unnecessary as there are gaming cards with six video outputs at a reasonable cost -- much cheaper than workstation cards: the Asus 7970 / 280X (from 2013) and the Asrock Taichi 5700 XT (from 2019), the latter allowing for six 4K monitors.