There is a new term called streaming, where two (or more) songs are laid upon each other to give rise to surprisingly unique harmonics. Some songs are so similar, they seem to be different only in the lyrics.
Take The Police’s “Every Breath you Take” and Ben E. King’s “Stand By Me”
or Ringo Starr’s “It Don’t Come Easy” and Eric Clapton’s “Let It Rain”
Or how about how one song’s finish can lead into another’s intro:
The Beatle’s “A day in the Life” leading into Pink Floyd’s “Us and Them”
What others can you think of?