chopjock,
Good question: Why do anything? Shutting down a dead engine is closing the barn door after the horse has gone, and shutting down the remaining engine is the beginning a a Very Bad Day, so why grab the levers at all?
On a twin helo, grabbing the engine levers during the early phase of an emergency should be a capital offense. CPs who do so are looking for a way to help, and must fight the urge.
First rule of medicine and piloting: Do No Harm
The aircraft is already trying to get your goat, don't compound it.
The CP's job is to be another set of eyes and another brain. Diagnose, discuss and then decide.
Even if you had a rare (almost vanishingly small probability) high side failure, that "problem is that you have too much power. As if too much power is a bad thing!