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I had the idea over the winter while pondering my SSI installation and I was just now reminded when I read Ed's post. In short, the idea is to run the stock alternator with a 12v regulator that charges the lower battery. The additional alternator could use an identical 12v regulator but be used to charge the upper battery. Has anyone tried this, or seen something similar?

As long as there is no grounded "zero" on the upper alternator/regulator it should work like a charm charging both batteries independently as needed. Don't have to worry about balancing batteries, but best of all, 12-volt loads could be pulled off of either battery without concern (as long as the alternator is capable of supplying the full load current). All the 24-volt wiring can remain unchanged.

While it sounds technically feasible, there are several issues with doing it that would make it hard.

First, running the 24v alternator at 12v would severely reduce its output power capability - which isn't that high to start. You could always put in a 12v VW or Porsche alternator though.

Second, I think of any isolated car alternators - but that's probably something you could modify to achieve.

I would say a simpler solution is leaving the stock unit as is and add a second battery and an alternator or Dc-Dc converter to charge it. This allows the truck to still start if the 12v battery is low (Although I am not sure of how low the SSI system will work at...).

FWIW, U.S. military Blazers (and I think Chevy pu's too) run a 12/24v dual alternator system. I don't know the specifics, but believe each alternator tends one battery. As I understand it, this prevents the disparity between the two batteries that often occurs when you only charge them with 24v.