Lots of good reasons mentioned here about why you have two batteries. I
like a starting battery like you have in your car for starting and a
deep cycle for other things like trolling motors, stereo, refrigeration,
anchor lights, etc. This is the kind of equipment that draws your
battery down farther and a deep cycle doesn't mind this as much as a car
battery. Car batteries are also cheaper.

I like to connect mine with a combiner (not an isolator) that you can
get at West for $70 and hook up in 15 minutes. It sends power to
whichever battery needs it without messing with that A/B switch. Im
going on 5 years on my set and they still work well. I also don't take
them out of the boat here in Minnesota (where it is 0 right now), but
leave them on a 600 milliamp maintainer instaed, but that's another topic.

Capt. Jeff