the two batteries I was referring to are main battery and backup battery not the two halves of one of those.

 

My point was, if you have two virtual batteries isn't that good enough redundancy without further being able to scrounge the good half of one of them (effectively making 3 power busses).

 

re: the alternators I meant lower mtbf than battery.  sorry.   but in practice if the alternator fails, you go to the battery.  So the battery is what counts in terms of keeping you from SOL.   Therefore, you could ignore the alternator MTBF.   ....or at least the improvement for having a "3rd" battery is unaffected by the alternator MTBF.

 

Colyn