The Collaborative International Dictionary of English v.0.48:

Duet \Du*et"\, n. [Duetto.] (Mus.)
   A composition for two performers, whether vocal or
   instrumental.
   [1913 Webster]

WordNet (r) 3.0 (2006):

duet
    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]
    5: (ballet) a dance for two people (usually a ballerina and a
       danseur noble) [syn: pas de deux, duet]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "duet":
   bipartisanship, both, brace, coaction, coadjuvancy,
   coadministration, coagency, cochairmanship, codirectorship,
   collaboration, collaborativeness, collectivism, collusion,
   commensalism, common effort, common enterprise, communalism,
   communism, communitarianism, community, complicity, concert,
   concord, concordance, concurrence, cooperation, cooperativeness,
   couple, couplet, distich, double harness, doublet, duad, duettino,
   duo, duumvirate, dyad, ecumenicalism, ecumenicism, ecumenism,
   ensemble music, ensemble singing, esprit, esprit de corps,
   fellow feeling, fellowship, harmony, joining of forces,
   joint effort, joint operation, mass action, match, mates, morale,
   mutual assistance, mutualism, mutuality, octet, pair, part music,
   part singing, part song, polyphonic music, pooling,
   pooling of resources, pulling together, quartet, quintet,
   reciprocity, septet, septuor, sestet, set of two, sextet,
   solidarity, span, symbiosis, synergism, synergy, team, team spirit,
   teamwork, terzet, terzetto, the two, trio, triumvirate, troika,
   twain, two, twosome, united action, yoke