Mustang 51 has a good point about the good engine following the failed engine rpm down towards idle. If there was, ,say a 400 -500 rpm dropoff differential, that would have to occur before the good engine executed its rpm decline, then seeing the plane slow down will be your first que that something is not right. Don't depend only on your ears, because there will probably be other planes flying. If, on the other hand, one of your 2 stroke engines was set a little rich, as soon as you get to altitude and back of the throttle, it could develope an intermitant 4 stroke and drop the rpms off 300-400. That is NOT a good reason for a power reduction on the good engine. You will probably recognise this condition, land and adjust your needles. Thus, the 400-500 rpm allowed rpm diferential.??? What do you guys think?