You don't need multiple frequencies, each receiver should have it's own unique ID (basically a number), the transmitter sends out both an ID number and a command, all receivers listen to each transmission, but only act on ones that use their unique ID. To send to all receivers, simply adress each receiver in turn.
It's how everything remote control in your house works!, and why your TV remote doesn't affect your VCR or stereo.
Check my PIC tutorials for examples of IR and radio transmissions.