While I don't have the 12V/24V issue, I run two 12V batteries in my VW...the stock 45A alternator is anemic at best, and when I demo'ed the stereo at car shows, I would run a single battery dry in no time!
So I have one battery for the stereo and the other for the operation of the other essential vehicle components (incl. the starter
).
An external regulator allowed the alternator to charge each battery seperately...otherwise, you put a tremendous load on the alternator...