Do I have to get 2 water cooling reservoir/pump/radiator kits to cool 4x 1080Ti's and an AMD 1920X Threadripper? Or, is it ok to put all of that in one loop? I have space in the thermaltake core p90 case for 480mm of fans/radiator. I don't know if the Core P90 even supports 2 reservoirs.
Is this another bitcoin miner?
Water cooling is an unneeded extra expense unless you plan on overclocking the crap out of everything. Plus I suspect removing the factory coolers from the cards & replacing them with water blocks will void the warranty. That's an awfully lot of money to put at risk. Regardless, I think you'd better have a look at the bottleneck calculator. I took a stab at your specs & it shows a 27% bottleneck with that CPU & 4 cards.
umm yes why does it matter?
"why does it matter?" It doesn't matter one bit to me. If you want to spend a ton of money on hardware that won't live up to it's full potential, by all means, go right ahead.