Running even 3840x2160 at high refresh rates is impossible in heavier games without cutting settings.
Doubled amount of pixels from two monitors would be even more so.
Already 2x 2560x1440 isn't that much behind one 3840x2160 in GPU load. (7.4 million pixels vs 8.3)
Multi-GPU isn't any solution to that, because it's been drifting dead on water for many many years.
GPU makers simply were tiring of having to do driver tweaks for every game.
And then DirectX12 made it job of game developers to get performance out from multiple GPUs.
And before buying any expensive graphics cards it would best to wait untill next-gen GPUs are out.
Nvidia's high end card pricing is at rape and robbery level.
2080 Ti is only 35-40% faster than Radeon 5700 XT, while costing toward 200% more.
And because of people just keeping asking more butt-syncing, Nvidia's next-gen prices might be even crazier if releasing before AMD.
At any of those resolutions GPU will be primary bottleneck without major cutting of graphics settings.
But Ryzen 3900X would handle background tasks like web browsers, possible team chats etc without problems.
Also in couple years could change CPU to Zen3.
