remember backing up, leave the rudders centered (as my brother always yells at me, Don't touch the f...ing wheel Dumbass!"), leave the throttles in idle, and steer with the fwd/rev shifters.
 
Its hard for an outboard guy to break the habit of turning the wheel.
 
Congrats on the Bertram!. One of my dreamboats. 
 
Someday!