making a new 8 pin din cable

Hello, I am trying to hook a second subwoofer to my a/d/s ph15 amplifier with a 642Csi a/d/s crossover and I've been told I need to modify the DIN cable to do so. Channel 5 and 6 are for subwoofers but i want to bridge channel 5 and 6 for 100 watts rms and also bridge channels 3 and 4 at 100 watts rms. So basically I need to make a din cable that takes the signal from channel 5 and 6 and splits it allowing it to be used on channel 3 and 4 so that i have 2 bridged channels running my subwooffers. So what are my options and what all do I need to do such a thing? Here is a diagram of the original DIN layout on the amp:

So what are my options here? thanks