There's no issue with this. Whoever told you there was doesn't know these cars.
The rear doors are the same on all 240s. They're compatible back to the early cars. You will find wiring differences and obviously trim and hardware differences- everything you need to use can be swapped over to any 240 door.
89 to 93 is virtually identical. There could be minor wiring issues if the 93 has any extra like heated mirrors- no such wiring exists in the 89, so you simply don't connect anything to it. If one car or the otehr is equipped with power mirrors, you would have to swap the mirror for whichever style you want.
Otherwise, both have the same PW motor, power lock switch, power window controls, etc. Speaker is probably physically different but that's mounted to the door interior panel, no big deal.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K