It has only 1 cable with 2 branches.
My initial idea was to use the 8 pin CPU connector with an extension, where I cut the cables at one end and just plug them in the board connector, but something went really bad there.
Now I have connected both branches of the same PCI-E cable and all seems to be working just fine - I was measuring temps with an IR thermometer for like 20 minutes.
Obviously I would prefer to use the CPU cable, but I would not risk it with these cheap quality extension cables.
Now I have to find a way to use it.
Could be that you had the polarity wrong. 8pin EPS have the 12v cables on the other side and those extension cables is for PCIE.
Because all the cables is all black on CX600m, its very confusing.