Question on Dual Batteries
I have two batteries and one charger as well. Rather than buy a charger built in to the boat I opted for the Guest Charge-Pro portable charger. It is a bit less money and amps than some other models but is still a 3 stage charger. The reason for this choice was to buy one that I could hook up to the batteries at home when they are out of the boat in off season.
On the boat I simply connect the alligator clips to the ground and to whichever battery lead connects to the battery I wish to charge. This eliminates the need to remove the battery box covers. Since the boat is on a finger pier with power I hook up the charger at the end of the weekend and disconnect it when sailing.
This is not an ideal permanent solution but works well on a small boat that does not have an inboard engine requiring a starting battery. I have never run either battery out even after crusising for a week at a time.