Anyone who owns a T-Rex 450 or 500 knows that one of the big downers is the short flights and long charge times.  I have a Thunder Power 610c charger which is capable of charging a 6 cell pack, however I only have 3 cell packs for my T-Rex 450.  I read a forum post somewhere where someone built a parallel balancer lead which let him charge 2 3s1p packs at the same time.  He had the right idea but the problem with charging them parallel like that is that the balancer can't see the true voltage of any one cell.  It can only see the combined average voltage of the 2 that are paralleled.  The only way to safely charge and balance 2 3s1p packs is to build a serial balance lead that combines the two packs into a proper 6s1p.  The charger can then see each individual cell and will charge up to 25.2v, balancing the cells to 4.2v each.

First, gather your supplies.  I ordered 2 Align-style 3 cell balance lead extensions, 1 Align-style 6 cell balance lead and a deans serial power lead from an online hobby store.  You can build the deans lead yourself or use whatever connector your setup requires.  So long as it's serial and is configured to allow 2 batteries and one charging connection, you'll be all set.  I used a wire cutter/stripper and electrical tape.  I recommend using solder and heat-shrink tubing but I did this in about 20 minutes in a hotel room using what I had in my parts box.