A duplexer is a series of inductance coils and capacitors that acts as a band-pass filter. The duplexer can be used two different ways. The duplexer combines the output of e.g. the HF/VHF transceiver and the output of the UHF transceiver so that both transceivers can be connected to the same antenna.
It also can be used reversed, e.g. one transceiver can be connected to two antennas.