Very dangerous to charge them in parrallel, should never be done, as Kane said. We use 6 packs in Big Nipper, 3x24 Cells & 3x6 Cells arranged in 3 sets of 30 Cells in parrallel in the robot. Even packs in parrallel will discharge differently even if matched if just the wire length is different.

I dont see anything wrong with charging them as 2 seperate 24V packs, we would charge ours at 36V if the charges could, and the 24 Cell packs were made up from 3 9.6V packs and have not been seperated since new.

I would recommemnd that you discharge the 12V packs to a set voltage level (Most Pro-peak chargers and other do this) then re-charge them, check the capacities are the same (should be with new packs) then connect them together to get your 2 24V packs. When you connect in parallel unless they happen to be perfectly matched one set will start to slowly discharge the other and if connected for a long time could damage both sets. We simply connect them just before a fight and disconnect them just after.