I have a 60 amp (24V) alternator on a belt replacing the stock alternator. In addition I have a 280 amp alternator driven by a drive shaft from the damper pulley. It is also 24V so equivalent of a 560 amp 12V alternator. Each is regulated by its own Balmar electronic regulator, these have an input to turn them on or off, with the regulator off there is no field current so it is effectively shut down. There is a toggle switch at the nav station to turn on one or the other. I routinely run both: if the batteries are down a bit I will run the large one for awhile until the charge goes below about 20 or 30 amps, then switch over to the small one. I do this mainly because the large one makes a whining noise (and the small one makes belt dust when run hard), but it has the effect of testing both routinely. A backup isn't a backup unless you know it works! Also this arrangement has redundant regulators, if one alternator dies along with the other regulator, I can easily swap the plugs.

I had to manufacture the drive shaft for the large alternator, I was worried about the torsional vibration from the 4 cylinder diesel killing it. It is an Electrodyne, which has only a metal disk as the rotor (no wire windings) but still... My first drive shaft had a Spicer U joint at one end and a Vetus rubber propeller coupling at the other. The U-joint vibrated loose in Maine and began making some noise, so I took it off and built a new one with Vetus rubber couplings at both ends. This has continued to work. It puts no sideways force on the crank but at full load soaks up about 15 hp or so.

Here is a picture of the first drive shaft as I removed it at anchor in Maine. The alternator is a little smaller than the engine and weighs a dainty 103 lbs .