I thought multiphonic singing was a hoax or trick.
Now I think about it, I must be confusing this with something else, because my impression was that multiphonic singing is essentially singing multiple notes simultaneously, so as to sound like a chord. I just can't imagine how this is possible. But then 'multiphonic instrumental' could hardly be anything unusual, by that definition.
Please educate us.
I don't know about vocals, but I know a guy who can whistle a triad (three notes) relatively in tune. I've heard pieces with multiphonics on flutes and oboes, they sound terrible, I'm not sure how they work. I think they do something to muffle the fundamental to the point where one or two harmonics are distinctly audible... but barely.