You will get every possible combination before this thread gets tired. Here is mine:
1) You want each battery to be the same so you can swap batteries each trip. You want to swap each trip, so that you regularly charge each battery
2) You must have a starting battery to spin that big block.
3) You don't sleep on the boat away from shore, you don't have a fridge, so you don't need a deep cycle battery.
I would get to marine starting batteries, and switch them each trip, living with the fact that you don't have as much time at anchor as a deep cycle would.
Or you can add a battery isolator or battery combiner, then you would not have switch batteries each trip. The isolator/combiner would charge both batteries every trip. Then I would get one starting and one deep cycle.
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