The Collaborative International Dictionary of English v.0.48:

Diplex \Di"plex\, a. [Pref. di- + -plex, as in duplex.] (Teleg.)
   Pertaining to the sending of two messages in the same
   direction at the same time. Diplex and contraplex are the
   two varieties of duplex.
   [Webster 1913 Suppl.]

The Free On-line Dictionary of Computing (30 December 2018):

diplex

    (From telegraphy) Two simultaneous
   transmissions in one direction.

   Compare: duplex.

   (2000-03-30)