The real issue is 98% of the flooded lead acid (wet cell) or AGM 12V batteries out there are engine starting batteries, design to put out a lot of power for a very short period of time. This is also true for Marine Dual Purpose batteries. They also want to be re-charge immediately after large power output.
98% of the flooded lead acid or AGM batteries are for use in golf carts, design to output less power for a much longer period of time.
On paper, two 12V batteries may look better, but you will get better life out of two 6V golf cart batteries and they might be cheaper.