What type of radio system uses transmissions on two frequencies?

A duplex radio system is characterized by its ability to transmit and receive signals simultaneously over two different frequencies. This system allows two parties to communicate back and forth easily, which is crucial in emergency medical services. For instance, one frequency can be used for sending messages, while the other can receive responses, enhancing the efficiency of communication.

In contrast, a simplex system operates on a single frequency where communication can only happen in one direction at a time. This is limiting in scenarios where instant feedback is necessary. The terms analog and digital refer to the nature of the signals being transmitted rather than the structure of the radio system itself, therefore they are not relevant to the specific question about the number of frequencies used.