How can people sing two notes at the same time?
Harmonic singing. Basically, they sing one rather low note, and then use a resonance in the vocal tract (which must be quite strong and narrow in frequency) to enhance the production of just one high harmonic of the low note. When it is much stronger than nearby harmonics, we notice it as a separate note. This question is also a bit subtle. We made some measurements of the frequency response of the tract and put an explanation on the page Harmonic singing. It is also possible to hum and to whistle at the same time. This allows greater independence between the two notes.
