The thing is, not all games are optimized for SLI. In fact, it seems to be fewer and fewer are; when they do become SLI compatible, the patch is far later in the lifespan of the game usually. For this very reason, I always advise against it. Even if you get say, two 1060s or two 1070s, you're going to pay close to the price of a singular 1080 anyway, a card that will only drop in price from here on out, allowing you to afford a secondary one later in life when it is needed.
Another common misconception is that SLI doesn't stack the VRAM, although I believe that most 1060s and 1070s have 8GB of it now, or at least 6GB. Either way, if you're super set on going SLI, two 1060s would do the trick.