Just go ahead and be sure the BIOS is their latest and the maker says they support more than one GPU.
But a 1080 under load has been spotted at 350 Watts so that 700 of your 850W PSU without more headroom for the rest of the PC.
Still usually fine to boot, you should see random BSODs under load but for now, let's hear what BIOS it's at and if the motherboard maker tells you it supports this configuration.
-> Be sure to test that each card works. A few folk found out they had a dead card.
I just upgraded my PC with a fresh Windows 10. Initially I started with 1 GPU installed to get everything up and running. When I installed the second GPU my system now will not boot up - won't post. When I take the 2nd GPU out the system boots fine. I think I should have enough power - what are you all's thoughts.
Corsair 760T Case with no additional fans added
Ryzen 5 1600x
Asus Strix x370-F Mobo
Cooler Master Masterliquid Lite 120
Samsung 960 EVO M.2
WD 1TB HDD
G.Skill Ripjaws V 16GB 2 x 8GB DDR4-3200 PC4-25600 CL16 Dual Channel Desktop Memory Kit
1 - ASUS GeForce GTX 1080 ROG Strix 8GB GDDR5X Video Card w/ Aura RGB Lighting
1 - ASUS GeForce GTX 1080 TURBO-GTX1080-8G 8GB 256-Bit GDDR5X PCI Express 3.0 HDCP Ready SLI Support Video Card
EVGA SuperNOVA 850 G3 850 Watt 80 Plus Gold Modular ATX Power Supply