Reply to question about an example for a Dual Alternator system.
From Dick Koehler:
Right off the top of my head, I do not have a schematic for a two alternator and two battery system. However, it is a rather simple circuit. Just design two independent alternator/battery systems and tie them together with an appropriate circuit breaker, that would be activated in the case that either system failed, so that everything (up to the load capability of the alternator) could (in an emergency) be run off of one or the other alternators. Think of a dual parallel system.
A few details. I would put one of the engine electronic ignition systems on each alternator/battery system. Ditto if you have electronic fuel injection. A common approach to all this is to use a regular alternator/battery system driven by a belt on a Lycoming, and a secondary 20 to 30 amp system off the vacuum pump drive (such as B&C). The example wiring diagram in the SAW Electricity Notebook that shows two batteries and one alternator (tiered buss architecture) could be a starting point. Just add a charging circuit to the "Engine Battery" buss (middle of pic). The logic for connecting the new "Engine Battery" buss would obviously also have to change with the addition of the second alternator.
One other thought: Trying to run two alternators at the same time on an electrical system is a bit tricky. You would need voltage regulators from a multi-engine plane that have circuitry in them that will balance the load between the two alternators. If you do not have these special voltage regulators, and run two alternators at the same time in one circuit, one will dominate and take almost all the load, while the other does little or nothing. All the small alternators that I know of that are designed to run off the vacuum pump drive do not have this paralleling feature, and so cannot be run at the same time on the same circuit of a main alternator. Hence, the idea of running independent systems, and only joining them together in the event of the failure of one system and its subsequent shutting down.