There isn't anything technically different with one 12 volt battery or 2 6 volt batteries. When a battery fails, it's usually one cell in the string, not the whole battery. The redundancy is in 2 12 volt batteries, I understand that. But my bet is that if one of the 12 volt batteries fails, you are not going to notice it quick enough to save the other battery from being damaged. What a 6 volt battery gets you is less plates that are thicker and therefore more rugged to the jostles and bumps of traveling. The highways are worse than the back roads these days too!
So here's what I do, since I've been running 6 volt batteries; I have a cut-over switch from my truck, which has 2 12 volt batteries. Either one can start my truck, so I installed a disconnect on them. Now, if my 6 volt series batteries fail, I switch one of the under hood batteries over to the camper and I isolate the remaining battery to serve the truck on it's own.
The bottom line is; 2 6 volt batteries can store more energy in the same foot print as 2 12 volt batteries and have less chance of failure due to their heavier plate design. It's much less likely that a 6 volt battery is going to fail compared to a 12 volt battery.
