Your code is (on both ends) just sending everything it receives on the serial interface out of the serial interface. So you have a loop even if send on the two boards with different carrier frequencies. You need a different interface (not the hardware USART) for your control commands (the input into the system).
The system you linked to uses just a one way communication, you describe a hardware solution for two way communication. Maybe I misunderstood your description of the hardware setup, then it might be a good idea to draw a schematics to show us what you've wired.