Answering this question is tough, but we face it with every SLI benchmark. In this instance, the question is whether or not there's better value in spending ~$600-$700 on a single GTX 1080 or $380-$450 on 2x GTX 1070s. Based on the AIB partner cards we know to be hitting the market, let's just make it easy and assume an average GTX 1080 price of $650, and an average joint price of 2x GTX 1070s at roughly $800-$850 – so that's a $150-$200 gap between the two main options. To be worth it, you'd want to feel like you're getting an additional $150 of performance out of two GTX 1070s.
In some games, such a gain is reasonably present. We saw upwards of a 30% lead over a single GTX 1080 in a few instances, which is no small jaunt considering the already impressive performance of the 1080. Still, though, SLI and CrossFire run the same risk they always have: Poor optimization earlier in launch cycles or a complete lack of any meaningful support in some games (like Just Cause 3, Assassin's Creed Syndicate, and Doom). Some of this will simply never be resolved – it's the way the game was made, and the devs aren't going to be able to do enough optimization to meaningfully impact performance. Some of the detriment is just launch cycles – nVidia tends to put out SLI optimizations over time, as do devs, and games can improve in overall performance throughput.
Inevitably, there will be times when multi-GPU configurations simply don't work well (or at all) and necessitate that the second card is disabled. That's never fun, but there are also gains – like in Mordor, with >70% scaling – that can counteract the occasional loss. Ultimately, it's up to you to look at data on a game-by-game basis and determine if SLI makes sense for you. If you're very heavily focusing on one game – maybe hoping to competitively play Infinite Warfare or BLOPS3 for a year or more – it could make sense to go SLI, with an occasional toggle for unsupported titles. If playing a wider swath of games, we'd recommend very seriously considering a single GTX 1080 instead – or waiting for the RX 480s to come out, and then seeing if AMD's claims of highly performant CrossFire framerates are valid across all titles.
Overall, it's still a mixed bag of results. Some games show less than 10% scaling – or even “negative” scaling, particularly with poor frametimes – and others show 40-50% (or, rarely, greater) scaling in SLI. Look through the data and make a decision.
That said, if you're the type who will be greatly upset every time SLI has to be disabled and you're left with a $400 brick in your PC (until it can be used again), maybe just avoid multi-GPU configurations altogether. A single card is certainly the “safe” and easy route, and the 1080 is a good single card as an alternative – and cheaper than two 1070s.