I don’t know if you have ever seen this before or like me if you believed it was possible for one person to sing two notes at the same time. Well the very talented Anna-Maria Hefele shows us in this live video that polyphonic overtone singing is possible and that every human being is capable of doing this.
Apparently, 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.
You can do this with different movements of your tongue, lips and jaw and by changing the shape of the resonant cavities of the mouth, larynx and pharynx to produce the harmonics (fundamental and overtones) of a sound wave.
Wiki states “Overtone singing—also known as overtone chanting, harmonic singing or throat singing—is a type of singing in which the singer manipulates the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out of the lips to produce a melody”.
If that all sounds too complicated, Anna-Maria Hefele shows you how at her site :- http://www.openculture.com/2015/10/anna-maria-hefele-presents-a-6-part-lesson-on-how-to-sing-two-notes.html