Will 2 6 volt batteries in series last longer than 1 12 volt?

Two 6-volt batteries in series will have the same voltage as one 12-volt battery, which is 12 volts. However, the longevity of either configuration depends on their capacity (amp-hour rating) and how they are used. If both setups have the same capacity, they will provide similar performance duration. If the 6-volt batteries have a higher capacity than the 12-volt battery, they may last longer.