2 1070 will definitely be more powerful than a single 1080ti, BUT....
Only if the game/application can:
- Support 2 way SLI
- Scale close to 80% or more for the second card
Also note that you will need a larger PSU such as 750w/850w to run those 2 cards. Hence also more heat produced.
A single 1080ti may not be as powerful as 2 way SLI 1070. But however you get a much more smoother frame variance, and lesser heat and power consumption. And guaranteed performance than 2 way SLI when game first launch.
It goes 2 way, it is either you go for SLI and pray for better drivers update and good game support. Or without much headache go for single card and yet works well even when the game first launch.
SLI usually have a lot of bugs when game first launch. Things like....
- Graphics glitching
- Graphics artifacting
- Low frame rate
- Frame variance all over the place
- Micro stuttering