Usually those dual sets are more for providing cleaner looks, and using two separate cables will provide cleaner power with slightly more cluttered looks.

it will work but puts extra load on the wires, you will get more voltage sag as the load goes up during heavy use. better to go with separate cables to maintain stable and clean current.

The GTX 1080 Ti FE only has One 8-Pin and One 6-Pin PCIe Power Connectors Not Two 8-Pin PCIe Power Connectors.
The OP Should Post the Part Number of the GTX 1080 Ti Graphics Card.

As what I posted above as well.
But I am also Pulling over 760 Watts on Two GTX 1080 Ti FE Cards ATM with Pecks up to 800 Watts.

Sajin

bcavnaugh
As what I posted above as well.
But I am also Pulling over 760 Watts on Two GTX 1080 Ti FE Cards ATM with Pecks up to 800 Watts.

Sounds like you're talking about total system load. Two 1080 ti fe card's aren't going to pull anywhere near 800w by themselves.

Yes and No, and I did say Peeks.
My Full System Load is 1010 Watts to 1100 Watts and Peeking at 1160 Watts.
But this is only as good as my UPS Shows me; CyberPower PR1500LCD Smart App Sinewave UPS 1500VA 1050W SNMP/HTTP Mini-Tower.
But then I am still Playing Around. Pushing Both Cards @ 2126MHz.
Pushed it so far my UPS Overloaded (1050 was hitting 1120), running 2 CPU (10 Threads) and 2 GFN 20 so I had to set to run only One GPU Task.
Maybe my 1200 Watt PSU is also not enough Power. I wonder what AIDA64 shows?
With Asteroids I cannot even push them passed 280 Watts on both cards, GFN 20 is really hard on Graphics Cards.
 
Next week sometime I will have my GPU's on it's own UPS and can get a Better Reading.
Although I can be hard to tell based on what PSU I plugin to the 6-Pin PCIe Power Connector.
My KP Rig alone Pulls 130 Watts from this 6-Pin PCIe Power Connector.