Just because it is an 8 pin connector on one side, does not mean it is for connecting to an 8 pin to CPU cable.  It could be.  Or it could be a PCIe power extension cable, which is could be 8 pins or 6 pins.  The 8 pin PCIe power cables are wired differently than the 8 pin CPU power cables.  Be very careful.

Based upon the OPs original post, it is not clear at all what he is working with.  Pictures of the cable and the 8 pin connector would help.  He said he ordered a PCIe Extension cable, but that he got a CPU extension cable with a y on it.  Never heard of CPU power extension cable with a Y output to 2x6 pin connectors.  That would seem to overload that line considerably.  I have seen 8 pin PCIe extension cables that Y out to 2 PCIe 6 pin cables.