Possible Clues:

• Piano piece with "primo" and "secondo" parts, e.g.
• Puccini's 'O Mimi, tu più non torni' e.g.
• Number divided by two?
• Singers expected first in theatre
• Something one can never do?
• 'Beauty and the Beast,' e.g.
• "Cinque, dieci, venti, trenta" in "The Marriage of Figaro," e.g.
• "Up Where We Belong," for one
• ''Say Say Say,'' say
• Piece marked "piano, four hands," obviously