I like Roycv's suggestion.
Also, what radio system and model are you using. If it is a computer radio, you should be able to set up mixes without any additional external mixer.

Hi that is interesting, I would use a mixer running from the receiver. Just 1 (up/down) channel dividing into 2 servos via the mixer unit would give planes up and down, with both working together. Then the left/right of the same stick to have the rudder input to the mixer for the banking. This leaves the right hand stick l/r for the real rudder control.

You just need one stick position for this set up.

I think that works, does it fit with the mixer unit you have?

Roy