Your 1080ti can pull around
300++ watts during ZEC mining , ( mining zcash need higher frequency for the gpu , not the memory ) you can underclock/undervolt your memory to save some power
Your 2x1070 , each one can pull over 200 watts easy ... so that is
400Watts ++your system , mobo/cpu/memory/hdd pulling around 60-
80 watts300+400+80=
780 watts and you have a 760 watts rated psu ... i would not use a psu over 80-85% load , so stay on the safe side 650watts total system draw is would be totaly safe if you have a no name , no silver/gold/platinum rated psu .
Do you using powerded risers ? Do you using both 6pin and 8 pin power connectors on your card ?