Considering a pair of ASUS TUF RTX3090 O24 Gaming GPUs.
SLI kind of fell by the wayside, and honestly, MSFS can’t fully utilize a 3090 as is let alone a pair of them. With DLSS support in MSFS around the corner, a single 3090 (which is already more than enough) will be even more so.
You would be better off with dual CPU.
Haven’t seen that motherboard yet, The Ryzen R9 5950X needs a co-pilot?
If you were doing a new build for MSFS only, you’d be better off with the Ryzen 7 5800X3D
I also run a 3090/5950X and MSFS is not able to utilize it fully as is, so adding double (either GPU or CPU) is not very advantageous.
I get some freeze frames in the game but otherwise fairly fast. It was stuttering terribly but maybe not so much as of yesterday. The ASUS Crosshair VIII Hero supports two GPUs but with no empty slots between cards. It will be hot even with external forced air fans.