so the solution is just use PCIE 6-pin splitter, one end for riser, and another one for gpu, so each gpu+riser only use one cable from PSU
My graphics cards take two PCIE connectors each and draw a lot of current. I'm not sure it would be advisable to use a splitter and have that one cable provide all the power for both the GPU and the riser.
If I can use molex I will use that over sata.