I am currently running two GTX1080 FTW cards in SLI. I cant seem to get both card to actually perform the same in game. I have the cards linked in the precision settings, SLI enabled in the Nvidia settings, however card 1 is only showing 253MHZ in the precision overlay. Card 2 is showing 1962 MHZ. It seems like card 2 is pretty much sitting idol. Please see screen shot. Is there something wrong with the card, both are less than a week old. I have not seen much of a performance increase over my 980ti's. 

Some games do not support sli.

Battlefield 1?

After the update the other day, battlefield 1 does not support sli.
You will get screen flicker in game.

Probably nothing wrong with the cards, have you tried other games? Like Sajin said, but also sometimes games are released without support, but later do. Also, people often make due by creating a profile in Nvidia Inspector I should mention that Nvidia isn't the Author of this software though.

Found this thread about BF1 problem that is updated. https://forums.battlefield.com/en-us/discussion/58545/major-update-flaw-flickering-blinking-updates-in-op/p1
 

Also note that directx 12 won't take advantage of your second gpu just yet.

DX12 uses dual cards different DX9-11
 
https://hardcoregames.azurewebsites.net/dx12-dual-video-cards/