im piecing together a build slowly. Either x299 i9 7900x or 8700k with 2 1080 ti's. water cooling loop for all.
Would 700w get it done ? I got a good deal on a used Silverstone 700w
The tdp of a non oc 1080 ti is supposedly 250w. Since you will be needing a decent cpu to not bottleneck them, I'd say your cpu would end up somewhere around 100w tdp. That is non oc, aswell. You shan't oc, but you shall be fine. You coult increase both of the ti's powerlimit by about 10%, getting them to 275w each, but that makes a total tdp of 650w, which is cutting it close, imho. Better stay safe, and don't oc. You'll probably have money to spare for a watercooling system or aio anyways -- if you're buying two 1080 ti's -- , which needs some power aswell.
TLDR
700w is enough with marginal oc. Better play it safe and don't oc.