Odyssey/Diehard platinum Parallax 7345 question.

You don't HAVE TO maintain an AGM battery with a charging device. They can be stored in a less-than-full state of charge and remain healthy. They are not like flooded cell batteries where a less than full charge may spell doom if left in subzero conditions.
If it was me, I'd use the small battery maintainer, paralleled to both batteries. You should probably think about replacing the Parallax converter as it won't be the best for battery longevity.