While wiring the batteries in parallel would increase the available amperage (and therefore power), I don't think that that is the goal. It seems like people usually want one marine style battery for running accessories with the motor off, and one regular battery for starting. This would require a switching system in some way that would ideally have the batteries in parallel for charging purposes.

 

Or, having one battery isolated from the other, with the high draw accessories (like a winch....) hooked to that one.... so, if you run it dead, you can still start/run the engine with the other....

 

BINGO! Thats what I want

 

That would be the ideal way to go. The only problem there is you have to somehow regulate the voltage going to the second battery from the Alternator so to not overcharge and eventually kill that battery. Several companies make regulators, but I'm not sure how expensive they are. It seems like that would put quite the strain on your alternator as well.

 

Something like this is what you are looking for.

 

Wire them separately and have them switched between starting battery (for starting), marine battery (for running accessories), and both (for charging and when not running accessories). I think that these switching mechanism are available.

 

I am just going to use a big relay like a ford starter solenoid and have it on a switch turn the switch on when I want to charge both battery's and for both battery start ups. and turn the switch when you don't want the two hooked together. The internal regulator will regulate the voltage. or you could get this there is no powerless with this as there is with a battery isolator http://www.powerstream.com/battery-isolator.htm