There is a BIG advantage to using 2 6v batteries in Series. That is paralleling batteries is never recommended. Batteries are never the same and do not age the same. The charging characteristics are mismatched, overcharging or undercharging becomes a problem. Overcharging can boil out the electrolyte, cause the battery to get hot and explode.
Considere series L-16 batteries used for solar installations, they are big. You don't get something for nothing.
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