Actually, putting the two packs in series would double the voltage. In parallel, as on a Y, the voltage would not change, the capacity would. If two packs were charged together that were not equally discharged, They probably would not charge evenly, one would either be undercharged or the other overcharged. If you post this in the batteries and chargers forum, there are some people over there that can explain it alot better than I can. But it general, I don't think it's a good plan.