750w high quality PSU can easily handle 2x1070 in SLI. The 1070 at stock speeds only consumes up to 180w, overclocked it may consume around 200W.
So yeah, high quality 750W psu will handle it easily, though again it all depends on the PSU quality, one of those cheap low quality PSU's will likely not be enough. I've had cheap Chineese PSU's in the past, in fact one was 800W and it died on my previous PC. Thankfully only the psu died and didn't damage any other parts.
Tomshardware have a very good rank of PSU's. Basically tier 1 to tier 2 are amazing PSU's, tier 3 is good and tier 4 is okay. Anything below that is crap and not recommended.