PSU requirement for dual GTX 1080

It'll really depend on the rest of your setup and the quality of your PSU. A Skylake CPU (~100W) + 2x 1080 turbos (360W) and the balance of your rig (call it 100W) is right on the cusp of a 600W PSU. If it's a quality 600W PSU, it should work, but wouldn't be recommended.

600W, even from a quality PSU is likely a little low though.
I assume, considering you only mention "600W PSU" that it's probably not the best quality.
 


I have the EVGA 600B
https://www.amazon.com/EVGA-BRONZE-Warranty-Tester-100-B1-0600-KR/dp/B00EON40CS

i7-6000k
Gigabyte z170 gaming 7
corsair h100i v2 cooler
2x 8gb corsair LPX 3000 mhz
1tb samsung ssd and 1tb wd blue
cooler master mastercase pro 5