If its a good quality PSU you might be alright
1080ti is recommended to have a 600w or 650w, and a 660 has a max tdp of 140w so in theory youre fine.
since you say the 660 will simply be a monitor output you shouldn't ever get near the 140w tdp of the 660, I wouldn't recommend overclocking them both and stress testing them both at once though.