The 2080 Ti can render at 8K at medium to high settings. While this isn't the same as 2x 4K, it does have the horsepower to run stuff like that, at medium settings. We're probably 1-2 generations from full dual 4K, but for now the top tier cards can run these units - just not at max settings.
P.S.- SLI/Crossfire for VR requires game engine support from the developer. Most VR devs don't elect this route due to cost constraints - there's nowhere near the gaming market for VR versus PC/Console. This typically forces them to cut costs to get the game to market.