There is no isolation, the 2 alternators will be connected to both battery banks, everything in parallel.
To my opinion nothing will be damaged by this.
Due to alternator voltage tolerances and cable length difference, the alternators will deliver a different current
. Seeing cable length, in general the port alternator will deliver more.
But there is no sense in starting both engines with the switch connected. Generally you'll see 80 amps or so running into the house battery. After a few minitus this decreases to maybe 50 A. So there is no sense in having both engines running, a single
alternator will already supply more than the house battery accepts.
That it does no harm us supported by the following:
Imagine your house battery is flat. You'll start your starboard engine
; then connect the emergency switch; then you can start the port engine, with the starboard still running. That brings you in the same situation.
, in the lifetime of our ship, I have already done a few hours with the switch connected. No harm done.