Don't quote me on this.... i don't actually think it has anything to do with the fact there may be two sets of vocal chords.
the whistling sound you can hear from each style of throat singing-
? Sygyt, (often called constricted throat singing) Produced by tightening the vocal chords and forming the mouth and tongue to create constricted airflow.
? Koomei, A very similar technique to sygyt, however it sounds less constricted and perhaps closer to the timbre western music commonly implicates.
? Kargyraa, The deepest,
- is in fact called a harmonic overtone and is the by-product of vibration itself, just the same as any of the instrument family, only it's more evident in chordophones and aerophones and less in idiophones and membraneophones.
The Tuvans learn to control the cavity of the mouth which in turn gives them control over the "resonant harmonic" of that particular "Fundamental" or "Tonic" ... or flippin root note for those who're not with me.
the reason I think this to be true, is that although to the untrained ear it sounds like the two pitched are independent of each other, the harmony (Whistle) is ALWAYS relative to the root in harmonic structure terms... there will always be 5th, 8th, 9th, 10th and 12th partial in the harmonic series in tune with the fundamental. the partials are called the "Harmonic scale"
Bow a cello and very lightly run your finger from the bottom to the top of the string whilst doing so...
guaranteed ...providing you're a listener rather than a hearer.... you will notice the harmonic relatives peeping out in a rather beautiful way.... these are the same harmonics....
there ain't two independent notes as you're thinking....
* relative harmonic
* modulation of fundamental and relative harmonic
* pitch change of fundamental (Thus creating new relative harmonics)
* Constricted / non-constricted techniques..
I must warn people about trying these ideas out too quickly....
Warm up or be warned!!!!!!! you do NOT have the capacity to sound like a 47 year old Tuvan who's has these techniques passed down to him from ancestors...and their ancestors etc, the very least you can do to protect your throat from nodules (lumps of painful knotting skin on your vocal chords) is to drink warm honey lemon mix and possible a voclazone tab to create a bit of antiseptic in your throat....
Go careful..... these guys are built differently to us. I promise....
Much love... hope it's been helpful.