As a singer, you can definitely hit two notes at once. This is something referred to as overtone singing.
Overtone singing also called overtone chanting, harmonic singing or throat singing simply means a type of singing where the singer manipulates the resonance created in the vocal tract in order to additional overtones above the fundamental ones being sung.
Basically overtone singing is a singing technique where one person sings two notes at the same time.
It is usually accomplished by manipulating the placement of your tongue and the shape of your mouth.
And in the end this produces a low tone and a high note.
Related: Check out this article I wrote showing you can a classical singer sing pop?
How to sing two notes at once
The sounds we make with our mouths generally involve two processes, the one that produces the sound and the one that manipulates that sound into something specific.
In speech, the vocal folds, commonly the vocal cords, vibrate producing sound waves from the air you exhale to talk.
This sound produced at the larynx, which is commonly called the voice box, is a sound that contains a spectrum of frequencies.
That spectrum is modified over the sound’s lifetime using the tongue, teeth, lips… all the things in your mouth and throat that are sometimes collectively called the articulators.
You can test this out pretty easily: sing a steady note and move your mouth from a wide smile into a narrow “o” shape.
Changing the shape of your mouth is enough to change the sound.
(Although, the same thing doesn’t happen when you whisper because whispered speech doesn’t make the vocal chords vibrate.)
Manipulating the sound coming from the larynx to produce two notes at once comes down to the resonances of the note or notes being sung.
Harmonic singing involves producing a note with the right frequency such that certain resonances — sounds in a system that tend to vibrate more strongly — are stronger than others, strong enough that it selects out a harmonic sound.
If that harmonic — a multiple of the original frequency — is amplified enough, we will be able to hear it as a separate note.
One way to achieve this two-note sound is with your mouth, like making your mouth opening small and tightly constricting your tongue and the roof of your mouth.
These kinds of shapes give you the resonance you need to find and draw out a harmony.
But a reverberant environment also helps, as does having a trained ear to pick out the harmonies.
Some singers are able to hold one note then manipulate their vocal tract to play up a resonance, choosing a specific harmonic from the sounds.
More skilled singers like Hefele can vary voice pitch and the resonant frequency separately.
How does overtone singing work?
To better understand this properly, here is a YouTube video showing you how this works.