At the moment SLi is shoddy at best.
Nvidia are currently switching to a new SLi system, and in case you didn’t notice, the 1xxx range of GPU’s lack SLi ports on the cards themselves. SLi methods will begin to be handled entirely by software in the near future, but right now it’s basically what I’d call “BETA”.
If someone wants to get SLi working on your game, they will get it working. Whether through Nvidia Inspector or other means.
I could be wrong but as it stands, I don’t even think UE4 has SLi support. I’m thinking an adoption of the new Nvidia standard is paused for now considering.
As for Crossfire, I am not educated enough to talk about it.