Two 6-volt batteries in series typically have a higher amp-hour capacity than a single 12-volt battery due to the total capacity of the two batteries being greater, assuming equal mass and technology. The design and manufacturing parameters of the batteries can also influence their performance. Heat dissipation may be slightly better with two smaller batteries compared to one larger battery, but this difference is usually negligible. If a notable capacity difference is observed, it is likely due to variations in battery technology or design. Overall, the amp-hour capacity should remain consistent across similar battery types.