I have a litte trick that helps me charge multilple packs a little quicker and still charge at 1C, but its kind of a vodoo process that's not really ideal unless you just happen to have all the gadgets laying around that I do. Ideally, you would use multiple chargers or a charger capable charging multiple packs. I do this also, but sometimes I have 8 or more three cell packs to charge and I can only charge 3 or 4 at one time with the chargers I have. I never leave charging batteries unattended, and I usually don't have several consecutive hours to baby sit the charge cycle, so heres a way that I cheat.
The middle cell on a three cell lipo is usually the one that gets drained the lowest and causes the balance cycle to take longer. Middle cell is also the cell that usually swells and fails first. Once I have all my chargers tied up, I "jump start" the next pack so it will charge quicker. I wire up the middle cell to an adjustible power supply that I have. I'll set it for about 0.25 amps of charge until a pack gets full and frees up one of the chargers. The pack in which I have "jump started" the middle cell will usually charge in about 30min when I do this. Like I said, it's not an ideal practice and probably not a good idea for most people, but it works for me.