I've done a similar thing, STDUN in my Funtana 40 (AXi 4120 motor). I have 2 x 2100mAh 4S packs wired in parallel. I made a 'clip whip' though you could also buy something like this comercially (FMA comes to mind). To do the clip whip properly, you have to be set up to do very, very accurate soldering or you could be in trouble.

The packs are wired such that I can disassemble the packs to charge them separately... from what I've heard, parallel packs should not be charged as a single unit. I use Sermos connectors.

If you need, I can provide a photo.

Bob