yea the issues arise trying to feed a series battery bank with multiple alternators. An alt knows one thing, make regulated voltage untill the load is so great it cannot. Without intelligent control they do not load share and will never balance. In fact the one way the two alternators will interact causes the battery imbalance situation.
12v standard reg voltage is 14.5v. 24v standard reg is I believe ~27.3V(13.65v per batt, better high hour high float charge voltage). If you feed these two into a series battery, the 0-12v batt sees 14.5 from the 12v alt. The 24v alt already being presented with 14.5v only inows to make up the difference to reach its regulators 27.3V setpoint. So the 12-24v batt only ever sees 12.8v and never reaches full charge…
now a standard alt referenced to ground and a second alt referenced only to the output of the first alt(wired in series like a battery) would probably work the best, as the two outputs would be reasonably close to each other. 14.5+14.5 =29V They would still however diverge with different loads applied. You would also need an alt with a completely chassis isolated ground which sounds like a pretty rare duck to me(probably custom or at least as rare as a reasonably priced neihoff)…
Again, absolutely pointless in todays world with 24-12 converters readily available…
one simple 24v power system, and you pull your 12v off of the 24 like any other 24v load.
@24v, the 12v load = ~15A. That is about as much as the truck pulls from 24v natively with trans, blower and wipers all pulling power…
if you are worried about redundancy, if the 24-12 converter fails you can still draw 12 out of the middle of the 24 to complete the mission. It will unbalance the batteries, but it will work to get you home. In fact if you rotate the batteries regularly, you could maintain this state indefinitely till you replace the 24-12 converter…