Whoa ! you cannot recharge 6v and 12v together. Period, end of discussion.
A 6v and a 12v have different AH capacities and different charge needs.
Any cart charger will supply amperage to the pack in series = all the batteries get the same charge. This will be a disaster meaning the low amp hour 12v will be overcharged and high AH 6v will be under charged! Don't do it
regroup