IMO, the best way to do it (if you are inclined or able) would be to pop open the ends of the two packs that you want to run in series and turn then in a "saddle" pack. I'd go ahead and wire up a balancing harness to both packs. That way you can discharge and charge both packs at the same time as one battery pack. Actually, you really wouldn't need to open up the ends of the packs. Just make sure that you have the balance tap wires straight and solder a new pack side tap to the existing wires.