Mate you MUST connect both the 8 pin connectors to the GPU. It wont work otherwise.
Is the PSU this one? If so, go get the extensions for the other 8pin connector. (I believe the red ones on the pic/video bellow).
Its an older version to that one and the card only has one 8 pin connector on it so surly the 6+2 should be enough or am i missing something?