Aaaaaannnnndddd - now you see why I run four 6TAGM's.
Two batteries work in a *perfect* world. They even work in a slightly imperfect world. They are better for the wimpy 100A alternator - no question. And two AGM's are better than 2 flooded cells. But if you lose a battery - it goes short circuit or something horrific happens and a terminal melts off it or it gets punctured, the alternator goes rogue and boils one before it catches fire (Yes I have seen all of these hypothetical scenarios as a mechanic of many years) - you no longer have the option to drop to two batteries from that position. Throw in a random voltage draw that crops up, something left on that shouldn't have been...... the possibilities are endless. This is why the nuclear power industry no longer considers 3-way redundancy to even be sufficient. The gold standard is now 5-way redundancy.
Four 6TAGM's are overkill. Till you are in the arctic and one of them dies. Then you still have a chance to reconfigure to two. You lose one of two batteries and now you have only 12v. That's definitely not going to get you moving.
It's a contingency thing for me. And an unwillingness to drop to two just because it's easier or cheaper or I gain space for some other component, etc. I'm stubborn like I guess.
