I guess that depends on the use case and how frequently both machines are running simultaneously. Like I said, that reasoning makes a lot of sense if you have a bunch of users coming and going, but the OP is saying it's two instances at most, so... I don't know if the math makes virtualization more efficient. It'd pobably be more efficient by the dollar, if the server is constantly rendering something in the background and you're only sapping whatever performance you need to run games when you're playing.
But the physical space thing is debatable, I think. This sounds like a chonker of a setup either way, and nothing is keeping you from stacking or rack-mounting two PCs, either. Plus if that's the concern you can go with very space-efficient alternatives, including gaming laptops. I've done that before for that reason.
I suppose it's why PC building as a hobbyist is fun, there are a lot of balance points and you can tweak a lot of knobs to balance many different things between power/price/performance/power consumption/whatever else.