A diplexer is a passive device that implements frequency domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other. Typically, the signal on port L will occup...
a multiplexer for either two transmitters or groups of transmitters, or two receivers or groups of receivers
A circuit or system that allows the ability to transmit and receive two distinct signals simultaneously. 
A device that combines two input signals into a single output. It may also be used the other way to