That is a great question. I'm looking for the answer myself. I would assume that it would last twice as long. My battery will usually last the better part of the day (About 6 to 8 hours on a charge.) I would guess that I could probrably get several trips without charging. I say, give it a shot, see how long it takes.
You will get longer running time by having them hooked up in series. Hook the positive to one of the batteries and the negitive to the other battery. When you charge them hook the charger to one of the batteries only. Hooked in series will it charge both batteries.
if i hook them in series wont that give me a 24 volt configuraton? I think my trolling motor is only a 12volt setup, MK pd40. then would i just hook the leads of the trolling motor to just one of the batteries?
Shakedown is correct in the wiring for 12V with 2 batteries (+)<->(+), (-)<->(-). I did just installed a PD55 on my boat a couple weeks ago and did that. I posed some pics in my OGF gallery for another guy I know that wanted to see how I did the install. As for charging, I installed a 2 bank minnkota charger with one bank connected to each battery. According to Minnkota, this is okay to do with the jumper wires still in place between the batteries and the charger will charge each battery without any problem. Unfortunately, I cannot really answer you as to the runtime compared to a single battery since I never had that setup. Although one thing I have been pondering doinging is adding one of those dual battery switches so that I can run both batteries at the same time or each battery seperately allowing me to use one while keeping the other as a spare.