Well thats never going to work. You're trying to put 700W+ through ONE cable which is designed for ~300W max. There isn't really a safe configuration in which you can draw the required power from a CX750M to a backpane.
Ok...so what would you recommend, then? A bigger PSU? One PCI-X for the Ground and one for the 12v? Something else entirely? There's got to be some way to power the back plane without the room sounding like it's got a jet engine in the corner.
You could potentially get away with using two separate PCI-E cables branches, but that's really pushing the connectors to the limit which is the problem. Ideally you'd wire up each blade individually, the backpane was never designed for normal PSUs and so you have 2 screw terminals trying to do the job of 10.
