Jugginator, SLI does not hurt performance. I ran two GTX 1070's in SLI using the high-bandwidth bridge and played 7 Days for quite a while that way and it helped. Unless this is new to A21, what you're saying is the opposite of my personal experience. I later sold the two cards and HB bridge to another member here and as far as I know it helped that user. I hope it did anyway.
I am trying to recall if it was 7 Days, but for at least one Unity game I played I had to do something special to get SLI really cooking like it does in Unreal Engine. I cannot remember which game or what, but at the time, Unity trailed in multiple GPU support for both AMD and nVidia. You may want to search for Unity and AFR (alternate frame rendering) and see if anything jumps out at you.
Update:
I did some searching. It appears that the Unity Engine never gained official SLI/Crossfire support. There are ways to make it work via nVidia Inspector or other external programs, but the benefits are NOT what they would be on an engine with native support. This tells me that it is very likely that I used something to make it give me a boost in 7 Days. I never needed SLI in Subnautica or The Long Dark and remember using only one GPU in those games. My main game at the time was Ark: Survival Evolved and it DID work there, granting a 50%-60% boost in FPS. Perhaps Jugginator is correct and, since SLI is now dead to nVidia, 7 Days cannot use it at all. Maybe sell the 1080 Ti's and get a 2080 Ti or 3070? I run a single 3080 Ti now and it roasts 7 Days!