diplex

from 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.]

from The Free On-line Dictionary of Computing (8 July 2008)

diplex

   <communications> (From {telegraphy}) Two simultaneous
   transmissions in one direction.

   Compare: {duplex}.

   (2000-03-30)