Probably only be able to fit 2 similar cards in a Mac Pro with the needed cooling. But that could seriously add up total specs with how integrated their chips are.
Space-wise, the Mac Pro can fit 4 double-slot cards (which fits all 3 models). Hardware-wise, the most immediate problem would be powering another 230-1400w of power. The other big issue would be bandwidth - the AX800 is a PCIe 5.0 card, and the Mac Pro chassis only supports PCIe 3.0 busses last I checked.
Frankly, I wouldn't bet on miraculous results adding Nvidia cards to Apple silicon anyways. Apple's compute hardware focuses very hard on saturating on-chip resources with bandwith, while providing comparatively small IO bandwidth. It's not a bad strategy for mobile hardware, but it can struggle to keep up with bespoke distributed systems.