The biggest question is "Can your alternator keep up with the draw?" If not, dual batteries won't help. You can wire the batteries straight parallel but the more common method for marine setups is to wire stereo and maybe accessories separate from the starting system and use a manual switch, isolator, or relay to prevent the starting battery from getting run down from the stereo running. In my setup I ran the stereo and a fuse panel block for things like ballast pumps on a second battery and it's wired parallel via a continuous duty solenoid to the starting battery. That way even if my stereo drains the battery, I can still start the engine but at the same time when the engine is running both batteries are being charged. As far as long term goes, straight parallel wiring is slightly better for the batteries since they'll both discharge and recharge at the same rate. I don't know if it's a big enough issue to worry about though. There are diagrams online for a wide variety of dual battery setups depending on budget and what you want out of them.
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