The Collaborative International Dictionary of English v.0.48:

Duo \Du"o\, n. [It. duo, fr. L. duo two. See Duet.] (Mus.)
   A composition for two performers; a duet. Duodecahedral

WordNet (r) 3.0 (2006):

duo
    n 1: two items of the same kind [syn: couple, pair,
         twosome, twain, brace, span, yoke, couplet,
         distich, duo, duet, dyad, duad]
    2: two performers or singers who perform together [syn: duet,
       duette, duo]
    3: a pair who associate with one another; "the engaged couple";
       "an inseparable twosome" [syn: couple, twosome, duo,
       duet]
    4: a musical composition for two performers [syn: duet,
       duette, duo]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "duo":
   both, brace, couple, couplet, distich, double harness, doublet,
   duad, duet, duettino, dyad, ensemble music, ensemble singing,
   match, mates, octet, pair, part music, part singing, part song,
   polyphonic music, quartet, quintet, septet, septuor, sestet,
   set of two, sextet, span, team, terzet, terzetto, the two, trio,
   twain, two, twosome, yoke