In article ,
says...

The alternator's field current is controlled by the regulator,
and when it's on but not turning the field gets pretty much full
battery voltage. That voltage would be a bit higher than normal if the
other alternator is running, but not dangerously so. The field can
take it.


Hmmm - maybe it's not the steady-state voltage - somewhat higher than normal,
as you say - that they're concerned about, but the transient due to inductive
field dump. When one is shut down, it could send a high transient down to the
other. If there is no specific transient protection, it could damage it.
That's just a guess too - I'd be interested in hearing more about it.

G Faris