RE: Eletronic engine syncronizer - no joke
Mine is not a Master/Slave unit. What mine does is has a single throttle input. It then drives both servos to that position (based on the included matchbox setup). Then after a very short delay it slows the faster engine to match the slower one. This will limit peak rpm on the best engine but it would be in sync at all times. It is currently designed with a second input channel for controlling the mode of operation (i.e. rudder or on/off was what I had in mind). I could add a third mode of operation controlled by the AUX channel to allow a three position switch to control whether the throttle stick controls port, starboard, or both engines like you are asking.
Also mine has a 4-digit LED readout (tachometer). If I eliminated that I could reduce the cost a good bit. If I eliminated that I could also add remote glow. Maybe that is the way to go.