I don't know if this relates to unbalanced or not but I was told that a charger hooked up to one battery connected in parrell to another will be over charging itself while charge is passing to the second battery. Hence, the recommendation to charge them individually. Another possible way to do it would be to mount a dual bank battery charger like I use on my boat. It is a smart charger and evaluates the charge on each battery as to when to go into trickle mode. For instance, my current charger is a variable 20 amp charger and will send more juice to the weakest battery until it has caught up.