Stick:

Just checking, have you changed the DX7 trainer mode to:

P-LINK: In Pilot Link the master transmitter maintains control of all secondary functions (i.e. dual rate, expo, gear, flaps, etc.) and only the primary stick controls (aileron, elevator, rudder and throttle) are transferred to the slave transmitter when the trainer switch is pressed.

That should solve the problem, plus you don't want to worry about trimming the slave.

Happy Landings!