Would that be brand specific or just a case of 1080 generating a lot more heat than 1070?
I'm pretty confident the same problem would occur with the GTX 1070 in SLI if you used two open-air coolers. With my mixed combo of open-air and blower cards, I had temperatures around 75C for the open-air card and 79C for the blower card on the bottom. And that was without having to do any major tweaking.
Remember, this testing has been done with a 240mm liquid CPU cooler keeping CPU heat out of the case, and the ACX 3.0 cooler on the EVGA GTX 1070 and 1080 cards I'm using is one of the best. Yes, you can spend another $30-$50 to get a GTX 1080 with larger fans and a much larger footprint, but they'll still run into the same problem, just not quite as quickly.
Now, since I posted my results, I've tamed the top GTX 1080 card with an ultra-aggressive 1:1 fan ratio (i.e., it runs at 80% fan speed at 80C, which is NOT quiet, while just barely maintaining that temp). The bottom card is running at the same clock speed but is hitting 69C with a 40% fan speed. So, you can definitely use two open-air cards, but it takes more tweaking using specialized GPU applications running in the background.