Shutting down alternators before engine shutdown

Hello All,
The manufacturer of the alternators in the Cessna 310 that I fly requires
the pilot to turn off the alternators before engine shutdown. Failure to do
this will void the warranty on the alternators. Does anyone know of a reason
for this? I have a suspicion that it has to do with the fact that with one
engine shutdown, the alternator's field on that engine can be energized by
the other engine's alternator. This may cause problems if the battery is low
(i.e reverse bias some diodes/transistors). That's just a WAG. Any ideas?

-Ali