You need to have both the 6 and 8pin plugged in at the same time for optimal performance. Some cards can run off only 2 6pin but you have to realize that the power differences between 6 and 8 is a 6pin is 75w and an 8pin is 150w. Only having 1 6pin plugged into the dual 6 to 8 adapter will only provide 75w when it needs 150w. You can run the risk of damaging components by doing that. Essentially, it'll be trying to pull that extra power from the 6pin.
I suggest purchasing one of these.
Athena Power Model CABLE-MPCIE4628 - Newegg.com
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