Wavelength Division Multiplexers or Demultiplexers (WDM) combine or separate two optical signals with different wavelengths. If we consider a “port” to be a physical SC connector and “circuit” to be a transmit and receive pair, the dual WDM module supports a 1310nm circuit and a 1550nm circuit over a common fiber pair.
For example, Telect part 056-1B00-6161 has six (6) ports or SC connectors on the front of the module housing. Two (2) of the ports are the common ports (labeled IN or COM), with each common port having a 1310nm output and a 1550nm output. So, one of the common ports would be for your TX and the other one for your RX on the common fiber pair.
Telect classifies the 056-1B00-6161 as a “Dual” WDM module because there are two (2) WDM muxes in the module housing. A sample application using two (2) 056-1B00-6161 would look like the following: