I've not done this on a 9303, but on a 10X I use one free mixer with a flap-aileron mix and a second mixer with flap-aux'X' (second aileron channel) mix and it works fine. Use the same offsets (I have the offset kick in at flap settings above 30 degrees) and mix % on both sides, if your aileron linkages and mechanical setup are identical this should get you very even deployment side to side (but measure and adjust both sides to be safe).

As long as you have two independent channels for ailerons this should work fine on a 9303 as well.

Barry