Possible Clues:

• Operatic love song
• "Happy Trails," e.g
• What a couple of people can play
• Certain recital piece
• Musical composition expected before tea
• The Everly Brothers, e.g
• 'It Takes Two' is one, appropriately
• 1967 #1 hit "Somethin' Stupid," e.g
• Work with a partner
• Outstanding first bit of tart -- a piece for two