Hi,
Most of you know I favor 12 volt jars, because they "fit" my Rv'ing style, but I have to say that if there are 2 twelve volt jars and one fails, the other isn't going to be much of a power source, unless the failure is noticed almost immediately, and the "bad" battery is taken out of the circuit extremely promptly.
I don't really enjoy "what if" questions. Maintain the batteries you have carefully no matter what voltage they may be and a good useful lifespan is the result.
Ignore the capacity and fail to care about charging with good maintenance? Then look forward to mid trip failures that are, at best, annoying and at worst dangerous.