The Collaborative International Dictionary of English v.0.48:
Secondo \Se*con"do\ (s[-e]*k[o^]n"d[-o]; It. s[asl]*k[-o]n"d[-o]), n. [It.] (Mus.) The second part in a concerted piece. [1913 Webster]WordNet (r) 3.0 (2006):
secondo n 1: the second or lower part of a duet (especially a piano duet)