Don't use 8-to-6 splitters, get a 2xMolex to 6-pin PCI-E instead and put that on two separate Molex cables.
RM750 and CX750 both come with two pcie cables that are branched-perfectly fine to use for regular speed at total 590W.
I checked connectors and cables temperature with a laser probe and they never exceeded 45C
You will just confuse the OP.
Sorry, I thought the poster meant some kind of aftermarket contraption to split 8-pin PCI-E into two 6-pin PCI-E. Corrected my post.