More batteries are always better, especially, if they power other stuff you use (pole saw, leaf blower, lights, etc ).Is charge time such that one batt can charge while other running the saw and therefore can continuously cycle w two batts or need 3 batts.
Realistically, you are rarely running the saw continuously: you make a cut, move, cut, reposition, cut, move some branches, etc. A gas saw will be idling that whole time, using fuel even when not cutting, whereas, a battery (or corded electric) saw stops each time you release the trigger.
As a practical matter you will probably be using the smaller, battery saws differently than a production bucker/ feller would use a larger saw.
Philbert