As a rule of thumb battery
capacity is related to the weight
of the battery
(the lead and acid).
The reason for using two 6 volt batteries in series is to increase the available amp hours and to still be able to physically handle the battery
A single 12 volt battery with 220 amp hr capacity could be made and may be available. The problem is that a single 12 volt battery with this capacity would weigh about 140 lbs.