No, I think the 2 receivers (pairs) can bind to the same transmitter. I think any number of receivers could, really.

Image now just taking a single Spektrum radio and bind it with a single receiver. Now get another receiver out and bind it to the same tranmitter. When you are up and running with 2 receivers (just like they were in 2 seperate airplanes), you are using the same frequency allocation when you just had one receiver.