I thought the only person that could harmonize one's own voice is Willie Nelson. If not, at least he adds to his singing chorus effects with his own voice!

Some talented singers. I sure enjoyed both singers with multiple vocal cords !!! In regards to the woman in the video who harmonized herself, someone said it's called polyphonic overtone singing. Come to think of it, I remember having heard Mongolians singing in this fashion though the woman in the video seems to be doing more than polyphonic overtone singing.

Here is Anna-Maria Hefele doing polyphonic overtone singing;