The controller's drive power is too high for all the current to be driven through only one set of cables. 120A x 2 channels is 240A continuous, well above the safe rating for AGW 8 wires if a single wire was to be used. When wiring the controller, you should wire the two thick positive wires (red) together to the positive terminal of the battery. You should wire the two think negative wires (black) to the negative terminal of the battery.