If you have 8-pin PCI-E, please take a clooser look as the 2-pins may be able to separate..
If not, you can use the 8-pin to 6-pin adoptor but the main thing is to make sure each PCI-E plug comes from its own rail. Not split off of the 1 rail.
Would an 8pin PCI-e to 6 pin adapter work?
If you truly have a 8 pin only PCI-E connector on the power supply, that means it can handle a 8 pin PCI-E to DUAL 6 pin PCI-E adapter if you wanted to because a 8 pin PCI-E connection is rated at much higher (twice?) wattage than the single 6 pin PCI-E connection (probably good if you want to overclock a single rail with single connection, but don't think it's enough to connect both separate rails).