Two different 1080's sure, say a gigabyte 1080 and an asus 1080. But you cannot SLI a 1080 with a 1070 or 980 or 1080 ti etc.
Benefits of SLI 1080s would be much higher framerates in games that proerly support SLI. The drawbacks are that few games especially more recent games support SLI properly or even at all. In those games having 2 cards in SLI could cause the game to not launch at all, have worse performance, run the same as it does with 1 card, or introduces various bugs and glitches.
I generally recommend single card setups over SLI nowadays. In my opinion the hassle of SLI is no longer even near worth the money of a second card.