The Harmonic kiss was conceived and developed by TRAN QUANG HAI some 40 years ago during his personal research on overtone singing .
One person makes the sound like a drone with rich harmonics while the second person creates the external resonator with strangulated throat to create a fixed resonator .The 2 mouths never touch to each other but they are very near (about 2 mm distance). With the variation of mouth cavity by saying A E I O U without making sound., the second person with created external resonator modifies the series of harmonics and the person who makes the sound can hear his (or her) own harmonics.
This method has been used for music therapy to solve the problem of marital conflict. Also it is used for the contemporary creation of harmonic kiss choir . Tran Quang Hai did experiment this method for a world premiere of collective harmonic kiss in France during his workshop in 2009.
https://www.youtube.com/watch?v=G99cFqw5q4w
Beautiful HARMONIC KISS by BACH YEN & TRAN QUANG HAI
TRAN QUANG HAI's world throat singing 