When you DO charge parallel like that, all of the number one cells are going to be averaged for voltage in the balancing read-out, so like Dave said, you'll need to be certain the packs are very close in voltage PER CELL otherwise you'll have problems.

i.e.- if you charge two packs, both #1 cell voltages will be averaged as well as both #2 cell voltages. So if one cell#1 is 3.86v and one is 3.8v you'll read 3.83v on the display and the charger won't know one cell is higher that the other... one pack might over-charge and one won't fully charge...