See: PCI Express Graphics Power Connector Maximum Power-Handling Capabilities

"Even though the specification allows for a delivery capability of 75 W (six-pin connector) or 150 W (eight-pin connector), the total power-handling capacity of these connectors is at least 192 and 288 W, respectively, using standard terminals, and even more using the HCS or Plus HCS terminals."

You can also use Y-adapters to convert multiple peripheral power connectors into a single eight-pin PCIe power connector, such as Dual SATA or Dual Molex to 8-pin PCIe power connector.

Exactly what power supply do you have?..If it's a multiple +12V rail unit, need to be sure the rail is rated for the dual 8-pin amps which 'on paper' 300W @ 12V = 25A..But a 1080 Ti FTW3 is a 280W card and can get up to 75W from the PCIe x16 slot itself.

The short answer is, yes, a 6-pin to 8-pin adapter should work perfectly fine.

Sajin
It will work, but for safety purposes I'd highly recommend using two separate pci-e power cables for the ftw3 instead of using an adapter to convert the 6-pin to 8-pin. Using a single cable to power the ftw3 isn't a smart idea imo as the ftw3 can draw up to 358w when using the slave vbios with the power target maxed out. Using adapters just creates another failure point as well.

That is what I was thinking, however this is more of a short term use fix.  I do have a 1000W psu, but it is a bit older.  I am planning on rebuilding my computer (more like building a new one) soon, and I really don't want to leave my GPU sitting around not getting used.  I guess I will try it out, and if it doesn't work then I will go with only one GPU until I get the rest of my parts.