Design,
If you look into the Twin Synch manual you will notice that there is an option to slave the port and starboard throttle servos to the rudder at (if I recall correctly) less than 40% throttle. When this option is selected on the Twin Synch board, at less than 40% throttle, as you operate the rudder, the left and right engine throttles will advance or retard and let you steer by differential throttle control (just like the full sized aircraft). Above 40% throttle (or what ever throttle % you choose) the rudder / throttle slave function is disabled and the port and starboard engines remain synched.

I know this option exists because my buddy flies a Ziroli Twin Beech with this option selected. Where once his take-offs were very exciting (the steerable tail wheel is fairly ineffective on a Twin Beech), the rudder slaved throttles tame the Beech takeoffs to a very manageable condition. This setup also helps him in taxiing the model and its kinda cool to listen to the throttles come up and down on different engines (like the real aircraft) while taxiing.

You can find more info on the Twin Synch system here: http://www.rcshowcase.com/html/acces.../twinsync.html.

And now the disclaimer. I am in no way associated with either RC Showcase or the Twin Synch manufacturer. I have however helped my buddy, a Beta tester for the system. If set up properly, I think the Twin Synch will get you what you want. I'm still not sure what to do with the middle engine throttle however, but at least you have a place to begin.

Hope this helps.

Regards,
langerl