Kwigen is correct, an 8 channel radio does NOT transmit on 8 different frequencies. Each "channel" is actually a "function" such as throttle, aileron ,elevator, rudder and up to 4 additional "functions" in an 8 channel system and all channel information is transmitted simultaneously. You can get an 8 channel system capable of transmitting on more than 1 frequency but at present this is not the "normal" thing. It is becoming more normal however.

You can buy transmitters capable of multiple "model memories". This means that you can set a set of conditions (mixes and servo reverses and throw travel limits) for each of several different planes. It used to be that you would need a separate transmitter for each plane.
That said, unless your transmitter is capable of changing frequencies all of your airplane receivers will need to be on the same frequency.