Possible Clues:

• Sonny and Cher, e.g
• Hall and Oates, e.g
• Hall & Oates or Sonny & Cher
• Wham! or Roxette
• See 20-Across
• Twosome of mainstream music
• Tegan and Sara, for one
• Sonny & Cher, e.g
• The Chainsmokers or Eurythmics
• Pet Shop Boys, e.g