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"1) Does it make economic sense to get two 75 amp Balmar-type alternators and to arrange them with two belts on either side of the block?Does "balancing" the physical pull on the PTO lessen wear?"
Yes, because of the pull/wear on the bearing. But that's making rash assumptions without knowing how much pull the bearing is intended to deal with. Generally, irregardless of what it is designed to take, a bearing will be happier with "balanced" side loads.
"2) Would I get better total output from two 75 amp alts or from one 135 amp?" Depends on your regulator(s). In theory you get 150A max from the first and 135A max from the second, but in practice, the first has some slight (very slight) extra load from two pulleys, belts, etc.
There's one issue that Balmar will tell you about right up front: Once you exceed about 100A in a single alternator, you can't drive it with a single V-belt. You need dual belts, and if you run dual belts without a belt tension guage, you will continually be busting the one belt that is really carrying the load--and then you need to replace BOTH again in order to get them balanced again.
You'd be better off doing what the auto industry has done, switch to a RIBBED BELT so there is only one belt. Of course, that may mean a custom pulley on the PTO and a custom pulley on the alternator (unless you can find a stock one that fits) at a price around $125-150 for each pulley from a machine shop.
Incidentally--buying custom pulleys can make very good dollars and sense in any case, because you want to match your alternator output to the engine speeds you will be cruising at, charging at, and maximum at. This can be more valuable than your regulator itself.
"3) Am I right in assuming that the Balmar-type alts are internally regulated, meaning I would not require an external Xantrex regulator?" Balmar offers all kinds of things, even alternators with dual outputs, each separately regulated, to charge separate/redundant banks. Give 'em a call or email them, if they aren't out to the boat shows they are VERY willing to talk and advise.
"4) Am I right in assuming that it might make better sense to have two identical alternators for redundancy of spares and for the fact that if one goes, I can still make amps, rather than for an "all my eggs in one basket" approach?"
Well, I like eggs in baskets, plural. But the folks who fly the big jet planes say the fewer engines you have, the better the chance that one of them won't fail in flight. They have a point there. Whether you go with one or two alternators, keep something on hand as a spare, even if it is "just enough to run things but the food in the cooler is going to spoil".
"5) Would there be any consequences if one alt died or required service, to substitute the old 55 amp alternator in the charge circuit with the 75 amp?"
Consequences...only if you got the wiring wrong. IF both alternators use the same type of harness (i.e. both are "3 wire" type) and both use the same regulation (positive or negative frame, etc.)...they can be plug-and-play interchangeable. You just need to make sure that the hookups for both will be the same, or that you do what it necessary so that one night, when you are seasick, overtired, and bouncing around in a storm, you can "just" unplug the dead one and plug in the good one. Preferably by just changing one harness plug, and putting one (normally not used) belt back on the engine.
Or by unbolting the dead one, replacing it with the spare, whatever works for your setup.
But dual belts...uh-uh, don't like 'em, that's too many kids in the same sandbox, adult supervision WILL be required.