Just checked my settings on my 12X so here you go...

Plug rudder servo into rudder channel, plug steering servo into aux4 channel:

In (17) Device Select inhibit aux 4,

In (5) Prog mix 1 mix rudder to aux 4, my travels set at L 100% and R 100%

In the same menu in Prog Mix 1 scroll to the box named gear and select either 0 or 1; the reason for this is to prevent the steering servo from operating when the gear is retracted. The 0 or 1 setting will depend on the servo reverse setting for the retract servo, the type of retract unit used, etc. Just flip the gear switch to gear up and see if the steering servo still operates. If it does just change the 0 or 1 value.

I have been running this setup for a while now and it works well. It will allow you to have individual adjustment for steering. You can use the aux4 sub trim for steering trim since you really shouldn't need to change the steering adjustment all the time.

Gary