2 SLI gtx 1050ti vs 1 gtx 1080

Solution
Currently only 1070 and up can SLI. Microsoft is working on multigpu but it's still a no go for most. Not to mention it takes 2 X 480's to match a 1080 and the 1050ti doesn't even matter h up to an rx470.
1) even if you could, you do not want to SLI. Multi-GPU isn't supported in every game, and even when supported you may get stuttering. Even if gaming is SMOOTH your FPS boost might be 80% in one game and only 30% in another.

(also the price means you would compare a GTX1080 to 2xGTX1060 6GB, not the GTX1050Ti)

2) Example: https://www.techpowerup.com/reviews/MSI/GTX_1050_Ti_Gaming_X/27.html

That's an average for SINGLE GPU, but if we assume a game has no support this is what you can expect roughly.

For example, the GTX1080 is 2.8x faster than the GTX1050Ti.