My old Futaba tranny although it has mixing capabilities also has no memories so if you fly several different planes it is not really practical to use.
My solution was to use one of these 'mixer' units.
Small (little bigger that the servo plugs) light and cheap you just plug the aileron and elevator channels from the Rx in one side, the two servos into the other and they are correctly mixed!
The two trim functions are not effected and it even uses a bit of the available servo over travel so you still have aileron control with the stick full back.
My Depron Skyray uses one.
It has worked perfectly, which is just as well as it is permanently built in within the fuselage!