The unit I bought is a Coolmax CU-700 , it appears to have a red PCI express 6 pin connection and two more 6 pins connectors for peripherals , could I use one of them for the second 6 pin cable?
The Coolmax 700 has two 6 pin PCIe power connectors.
The 2X2 (2 yellow wires and 2 black wires) and 4x2 (4 yellow wires and 4 black wires) connectors are for CPU power. They cannot be used to power a video card. The connectors are not supposed to fit PCIe power plugs. And if they did, you would be shorting 12 volts to ground because they are wired differently.