The only way to keep the systems seperate but still have a common charge point would be to modify your charge jack... You'll have to rig up some sort of "reed switch" or something like that on it, so that when the charge jack is plugged in, the two battery packs are bridged parallel to each other. Keep in mind by doing this you will be greatly increasing the "size" of the pack your charging, technically you should really then charge with higher current, but if not, you will need to allow for the extra charging time.