An issue that hasn't been mentioned is the Peukert Effect.

The slower you draw power out of a battery, the larger the capacity of the battery is. That's why all the capacity measurements are made at defined currents.

Why is this relevant? Because if you place two equal batteries in parallel, the combination will last longer than the sum of the two batteries individually since you are drawing half the current from each.

There are several reasons not to connect in parallel as well. Discharging one battery into the other has been mentioned. If one goes bad, the other will probably go too unless you notice right away. The requirement that they be matched is important too.

One of my favorite solutions is to add a second TM connector to the bow of the boat for the second battery. It's cheaper than a switch, avoids some of the issues of parallel batteries, and is easy to change. Fish until the first battery slows down, swap the plug and you know you are halfway done with the day.