I have just changed from 2 ea D34 Optimas in parallel on a 12 volt motor(110AH) to the same 2 Optimas in series for a 24V motor.(55AH)
I haven't been in the water yet so I can't comment on how much motor time I will get with this new setup. Obviously, the larger motor will have greater amp draw at top speed but probably lower amp draw at slow speed.
BTW Optima D34 Yellow top is the same as D34 Blue top except on the Yellow top you have to add the screw terminal adapters to have screw terminals.