In DX12 with SLI enabled both of my gpus are limited to 50% usage but DX11 has no such issue, both cards are used 100%. I have 2 Gtx 1080ti's in SLI. This happens in Hitman DX12 and Rise of the Tomb Raider DX12 too.
Frostbite engine? I saw a guy on youtube do fine with 1080 SLI.
Maybe a 1080ti driver issue?