The Canadian pop duo Tegan and Sara Quin have been in the public eye for more than 25 years, during which they’ve released 10 studio albums and sold more than a million records. But for nearly 15 years, they’ve been quietly grappling with an elaborate catfishing scheme that’s changed their lives, careers and relationships. Now, they’ve decided to go public about it in a new documentary, “Fanatical: The Catfishing of Tegan and Sara,” which explores fandom, parasocial relationships and celebrity. Tegan and Sara join Tom Power to share their story.