The 8103, 3810 in Europe, had 6 mixes. 1 & 2 are multi-point mixes without trim. 3 & 4 are the plain vanilla mixes. 5 & 6 are trinm mixes. If you need the master trim switch to trim both channels, you'll need to use mix 5 or 6.

As has been mentioned, if you are using 2 channels for ailerons, this is built in as a Flaperon Wing Type in the Set-up menu. Plug the right aileron servo into channel 2 and the left aileron servo into channel 6. This will give you ailerons and flaps with trim on both surfaces.