Hey Andrew
There is probably a plethora of ways to do it.... Honestly I think Gary's method would be best for you if you are a little unsure...
The anderson connector can be disassembled by pushing a small flat head screwdriver though the back of the plug, you will see a 'tang' splade that locks in the anderson terminal.
You could cut 2 completely good leads and put another plug on there, what I would do is just cut 1 plug off a lead, then strip a centimetre or 2 in a place on the good lead to solder the second one to. So then you keep 1 good lead, and have a spare connector. No need to buy another Anderson connector.
Make sure your soldering iron is 60+Watts >> ideally 80W for that gauge wire. Or those marrets Gary suggested would be fine.
I made a new power cable (10AWG) that splits into 2 x 12AWG as with the single GM 12Ah battery pack you WILL have quite a voltage drop with the standard wiring.
The shorter you can make your dual drive power cable with a single GM battery the better.
Ooooh someone is close to tasty dual drive action