I.E. an overspeed on one engine. You than need to pull the collective underneath your armpit, to keep NR on the clock.
Killing that engine, i.e. with fuel cut (firebutton) will result in a massive decay of NR, if you don't drop the collective rapidly, cause the remaining engine just can"t spool up that quick from 0TQ to 120% or so, which would be needed, to save the NR with a high pitch setting.
Lots of things have to go wrong for this to happen.
1. You need an overspeed for a start
2. Overspeed protection has to fail.
You will be doing the wrong action by removing your hand from the collective and then moving it to a fire button which raises the issue of pushing the wrong one. Why not just roll back the throttle on the over-speeding engine?
Methinks you should review your SIM procedures!