The RDS8000 will buddy box with a wide variety of other Airtronics transmitters. You can buy a VG400 buddy box with chord from Hobby People at a very reasonable $29.99:

http://www.hobbypeople.net/gallery/479300.asp

I bought a second RDS8000 to use as a buddy box and "backup transmitter" and to get an additional two receivers. If you buy another full radio system, you just need the Airtronics buddy chord:

http://www.hobbypeople.net/gallery/477100.asp

At $15.00, it's an inexpensive add-on.