As someone who has had SLI GPUs dating back to the 3dfx Voodoo2 days of the late 1990s, don't bother. Ever increasingly, new games are starting out with poor or zero SLI (or AMD Crossfire) support. This doesn't take into account the increased fan noise and heat output from two GPUs which for me means a lot in an air cooled case, especially in the warm/hot months of the year.
By "poor" I'm referring to poor scaling, or the second card yields less than 75% increase in FPS improvement (in a perfect world, SLI of the second card should be a 100% FPS or double improvement). By zero I'm referring to either never going to have it or a patch coming later to allow multiple-GPUs (which often means crappy scaling).
I'm not sure what happened in recent years, but developers seem to have either gotten lazier or just decided that coding for multiple GPU support wasn't worth the additional work (or both). Get a single 1060 6GB and you will be good for several years at 1080p. It performs about like a 980 and is more than twice as fast as a 1050.