This has literally been a running list I've kept in my head for years and now I'm sharing it.
  1. Both artists have about equal singing time. (Example of song that violates this rule: "half of my heart" that Taylor swift and John Mayer "duet." had no idea Taylor was even in it until I heard it like 40 times)
  2. Both artists are about equal in talent. I know that one's subjective, but it's my list. (Example of rule violator: "lay me down" Sam smith and John legend. Sorry Sam smith. You just can't compete.)
  3. The voices should sound different! (Example of rule violator: "just give me a reason" Pink and Nate Ruess. They sound exactly the effing same)