The only reason the second 8pin is there is to provide more power than usual. Most of the time, on air, you won't even be able to overclock a card high enough for that extra power supply to make a difference. One thing that lots of people have done yet probably don't even remember is flashing a 6950 to a 6970. For most people, this ran flawlessly, even though a flashed 6950 has 1x8 + 1x6 and a real 6970 has 2x8. I've also tried this out with my GTX 660Tis, an MSI model that had 1x8 + 1x6, and a Galaxy model that has 2x8. It made 0 difference in overclock ability and/or stability to use only a 6 pin instead of a second 8 pin. Do you honestly think that any 770, even with double the VRAM, NEEDS 2x8 when even a stock 780 doesn't?
But w/e, don't try things for yourself to figure them out, just go on conjecture.