Interesting tech. It's going to be quite a while before VR really gets the horsepower it needs to drive two 4K panels @ 90 Hz. And neither company seems to be approaching SLI/Crossfire/mgpu left/right eye rendering. So this is the only option.
I keep hoping other companies like Valve hop onto this tech
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.