This could qualify as one of those rare dumb questions, but are the voltmeters on twins always supposed to move in perfect unison? I would have thought each alternator would be dedicated to charge one battery - moving exactly together all the time tells me the alternators are somehow combined?

I normally run the Perkos in ALL, but even with Starboard in 1 and port in 2, I still get the same. Maybe I don't know the operation of Perko switches, so I'm going to read up on that.

But, shouldn't the voltmeters operate independantly so you can observe the condition of each alternator separately?

It is rare that dual volt meters are EVER wired correctly. For the most part only one is needed. They will show the same if your switch is in the all position. So take one out and put something cool in there like a booste guage