If you have the plane trimmed the way you want on the master transmitter, leave the sticks centered on both and push and release the trainer button on the master. If the control surfaces move, adjust the trims on the slave until the control surfaces don't move as you push and release the trainer button.

Otherwise, you could have whoever is flying on the slave box trim it out when the plane is under their control, but since the person on the buddy box is generally a new flier... Probably best to have it trimmed for them.