I see no reason why a 24 volt alt could not be externally regulated to
produce 12v as an option on a switch, except for the catastrophic error
potential introduced.

This implies charge management by the skipper, and introduces
possibilities for errors that could prove dangerous, but which could be
managed by relays.

If you have more 2 or more 12 v batteries, you could wire 2 in series
and charge that from 24 v. This could cause charge imbalance if they
are not alternated as "tops" and not reconnected for 12v discharge in
parallel, or if not similar enough.

You would need to reconnect them to use both of them for 12v in
parallel, after stopping charging from 24 v.

Relays could be wired up to do this, and it was all discussed at length
about every 2 years ago.

Better to get one alternator each of each voltage and run the systems
seperately, sharing only ground and possibly a drive belt. Then you
would want 2 spares.

2- 12 alts could be connected so that one of them provides 24 v out,
but only if you can manage a non conductive mounting, or isolate the
12v output "ground" of the "top" alternator from it's case. It would be
possible therefore, to carry one spare that could replace either, if
one should fail, and charge maintenance could revert to an automatic
mode.

To evaluate the convenience factor, it would help to know the duties
and loads on the 2 voltage systems, i.e. 24 v starter and 12v house?
Or, do you have a 24 volt windlass or spotlight only?

It may be convenient to use a small 12 v generator in series with the
12v battery to charge the 24v battery, if it gets only occasional use.

most simply, a switched or relay setup could give you 24 v to start,
for an instance, and 12 v normally, with only a 12 v system and 2 or
more 12v batteries.

Possibilities abound!

Terry K