The it works is the engines will be in sync (i.e. +/- 25 rpm difference). The engines will follow each other down to the idle point. So the scenario you discribe will not happen. If you throttle up and one engine is rich then lean engine will be set to the rich engine's rpm. If you throttle back and one begins to four stroke the lean engine will be throttled back to match it. The only time the good engine is throttled back to the programmed idle point is when RPM of the other engine drops below 2000rpm. So an RPM differential does not need to be set since they will always be in sync.

The way the algorithm makes this work is when the stick is moved it moves both servos to that point then syncs the engines.