[jhautz]

You can do that as long as the 2 3s packs are balanced to each other. Meaning all 6 cells are the same voltage to start. If not you will overchagre the pack with the higher starting voltage. Most balancers have the ability to drain off small variations from cell to cell in the time it take to charge a pack, but if they are charged (or discharged) to different levels beofre starting you will likly ruin you packs that way.

The only time you should charge packs on the same circuit, wired in series is if you balance them together and treat them as a single pack, charging and discharging them together (like a dual emaxx pack setup)