overtone singing is a voice technique where it seems like one person sings two notes at the same time.
You can sing the overtone scale on one fundamental. Another fundamental has its own overtone scale, so in order to have more overtones to sing nice melodies, you can use different fundamentals and change them while singing.

the video is live, and every human being is able learn overtone singing 😉 ….well…not starting directly with polyphonic overtone singing…. but singing overtones on one fundamental isn’t too hard.