If there was only one other source of charging on board I'd agree.

But there's THREE other sources of charge.

Many single engine boats only have ONE source of charge - period!

I've no intention of throwing the alternator out. Instead I'm going to have it gone through top-to-bottom, replace the seal and stash it on board. This is even better than one "in the engine" in that it will receive no wear this way, yet it can be easily swapped for a failed alternator in the OTHER engine.

I also intend to (probably this coming winter) re-wire the starting system so that both engines start off one bank, and the house is completely separate. I already have my combiner on board to make sure that charging is done correctly.

I MAY put belt-drive alternators in. I'm torn between doing it and not. There are arguments for both sides of that coin and its not clear what the right decision is. If I do put them in I will have to fashion guards out of SOMETHING; during maintenance it would be quite possible to get into it while doing things like setting the rack, and that would be an extraordinarily bad day for you if you did.

With the one alternator out I need to lose BOTH the engine WITH an alternator AND the genset to have no sources of charge (if the AC charger fails I still have the genset's alternator), and if that happens I still have two fully-charged battery banks to run instruments long enough to get to port.

I don't see the risk here; if I lose an engine offshore I'm already headed for the nearest place I can get the parts to fix it.

The gear-drive alternator system is a bad design. Most people I know who had it have thrown it out and mounted belt-driven alternators. The alternators like to fail as they are right between the turbos where its nice and HOT, the oil seals leak and make a hellish mess all over the gear bellhousing and if the coupler fails or the alternator seizes (the coupler is just a black plastic disc) the pieces will go into the gear train and can shred the engine. You can buy FIVE Balmer alternators for the price of ONE of these Leese-Neville things, and a belt-drive alternator that will work, plus a replacement belt is available anywhere in the world in a pinch. This is definitely not true of these gear-train-driven monstrosities.