When my buddy was working with me I was connected to his DX6i. The way I remember it, only his radio transmitted to the plane, so he would set trim on his radio only. When he held the trainer switch control was given to my TX.
I honestly don't remember trying to adjust trim at the time since I was pretty busy on the sticks! I'll be in contact with him in the morning so I'll ask him what the skinny is. He does a lot of training duty in the local RC club, so I'm pretty sure he knows exactly how it works.
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Tony, good point - I do remember that he would switch back an fourth several times and make some fine adjustments prior to take off to be sure that the TXs were in sync! Funny how you forget the small details...