I run i7-5960X and 2 x NVIDIA Titan X in SLI. With NVIDIA utility I can usually select SLI or non SLI mode for each app but with MSFS 2020 that was purchased from MS Store, like i did, there is an issue accessing executable for MSFS 2020 as it is stored in some weird hidden protected folder, there is separate posting about this on AVSIM forums. Short of removing one video card, I can't really test MSFS 2020 on single GPU. I was eyeing to purchase new NVIDIA 30XX series card so I think I'm just going to wait for that now.