Video multiplexers are designed to allow recording of several cameras onto one recorder.

Simplex:

A simplex machine is best suited to applications where recording only is the priority. Simplex machines cannot display multi screen images (i.e. quad, 9 way and 16 way split) while in the record mode.

Duplex:

If an operator is monitoring the system (i.e. security guard) then a duplex machine is more suitable. A duplex machine can provide screen splits and user selectable images without affecting what is recorded onto the recorder. Should you use two recorders, it becomes possible to record and playback simultaneously.