The simplest, least expensive, and most common buddy box set up for a DX7 is the buddy box cord going to another JR transmitter. You do need the cord, and then have to make sure the control set up on the buddy box matchs the settings (servo direction, trims, etc.) of the master transmitter.

That said, there are wireless buddy box set ups offered by third party vendors. They are more expensive than the cord, but give you the freedom to move around. Personally, I think you'd want to be close to the person using the master transmitter so the two of you can let each other know who's got control of the airplane.