All right, I ordered a 9xr a while ago, and I finally got it a few days ago. I tried to buddy box with the stock firmware on both transmitters (9x and 9xr) but wasn't getting the channels to match up well. Upgraded my 9xr with Open-TX (I think it is the same as open 9x) and works like a charm.
Steps to Buddy Box:
Turn on main transmitter (my case the 9x) bind to receiver, plug in audio cable, without turning on the slave transmitter (the 9xr this one does not have a module in it) plug other end of cable into transmitter (powers on automatically). Had to set up trainer mode in master transmitter (9x) set what channels I want my student to be able to control. When you want to control the airplane ( you the teacher) you pull back the trainer switch, when you want the student to be able to control it you just let go of the switch (my opinion it should be the other way around). All'so, what ever channel you don't let the trainee use you have control over that control all the time.
Hope this helpful to others.
God Bless,
DavTixRC