The spektrum wireless buddy box allows any DSMX and DSM2 complaint transmitter to slave to to new DX6, DX7 or DX9 wireless trainers. The "student" radio just thinks that the "instructor" radio is the plane.
One of the best features I've discovered with the wireless trainer is that you have the option to set automatic instructor override. That is, instead of having to hold/release a trainer button, you can set it up so that as soon as the instructor moves a control, that radio takes control.
It's a really simple but innovative idea.