using airtronics m11 with 2 steering servos?

i have a 5th scale gas truck that runs 2 servos for steering and 1 servo for t/b

just got m11 yesterday, do you stick the extra steering servo in the 3rd or 4th channel and how do you get it to mix with the master steering servo

my dx3r was a piece of cake and this c-mix on the m11 makes no sense.

thanks