Possible Clues:

• Tune for two
• Two performing together
• ''Dueling Banjos,'' for one
• Number one can’t do
• ''Baby, It's Cold Outside,'' e.g.
• Musical piece for two performers
• Performance that takes a second?
• Something one can never do
• Performance by company, perhaps, expected ahead of time
• The Human League's 'Don't You Want Me,' e.g.