Guys,
Correct me if I am wrong. If you connect positive to positive, and negative to negative, the batteries will be in parallel. You will now have 12 Volts with addition of the two current amps available on both batteries.
If you have the space, you can run a 12 volt 55 lber at low to medium all weekend long on two batteries. A good onboard dual changer with 10-15 A output can charge both batteries overnight easily. New batteries every 3-4 years guarantees you have good batteries. Also install a dual Altus battery monitor so you always know the levels of your batteries. Keep battery water levels up and keep battery connections clean. Sounds like alot of trouble. Most of us don't get alot of time on the water. Good batteries systems on you boat ensures you enjoy the time you do have.
Good Luck,
BATMAN