I was watching Jeopardy and the clue was about one voice singing two notes at the same time - overtone singing. I'd never heard of it so I googled it and here's an example. How useful it is in real songs is debatable but it is quite amazing.

Check out starting at about 3:18 on the link below - that's where it starts getting really interesting.

https://youtu.be/vC9Qh709gas