No one has mentioned this yet so... Here's an alternative ($$) in case programming help doesn't come thru
The Twinsync will do what you want in addition to keeping your engines in sync.
I have set the twinsync to use a three position switch on the transmitter. Position 1 controls the left engine fully from the throttle stick while the other engine idles. Position 3 does the same for the right engine. Position 2 controls both at the same time.

With both engines running moving to position 1 or 3 allows full control of one engine while the other goes to idle. No mixing is required in the transmitter.

You might go to RC Showcase and download the manual, and see what other options it has, like dropping the running engine to idle when one quits, then allowing full control of the running engine.