The Collaborative International Dictionary of English v.0.48:

Duad \Du"ad\, n. [See Dyad.]
   A union of two; duality. [R.] --Harris.
   [1913 Webster]

WordNet (r) 3.0 (2006):

duad
    n 1: two items of the same kind [syn: couple, pair,
         twosome, twain, brace, span, yoke, couplet,
         distich, duo, duet, dyad, duad]