Apparently it is possible but takes years of training:
There is a vocal technique used by Tibetan monks which allows them to
literally sing a chord, or three notes simultaneously. This technique
is not considered appropriate for most styles of western popular
music, and therefore is somewhat of a mystery to many in the west. The
technique when properly done has a pronounced droning which sounds
similar to a digital audio effect. The technique may take years to
master, but can be mimicked by producing a low sound from the back of
the throat, and then shaping the tongue against the roof of the mouth
as if sounding out the letter R. Adjust the shape of the mouth
slightly until you hear harmonic overtones which are two octaves
higher than the note being sung.
I have sung in various choirs throughout my life and have never come across anyone who did this or any music scored for it so I am guessing it is very unusual.
It is likely to refer to the false vocal cords explained here:
The vocal folds ... sometimes called 'true vocal folds' to distinguish
them from the false vocal folds. These are a pair of thick folds of
mucous membrane that protect and sit slightly superior to the more
delicate true folds. They have a minimal role in normal phonation, but
are often used to produce deep sonorous tones in Tibetan chant and
Tuvan throat singing, as well as in musical screaming and the death
growl vocal style.
Full wiki entry
The false folds are also called vestibular folds and ventricular folds.
The vestibular fold (ventricular fold, superior or false vocal cord)
is one of two thick folds of mucous membrane, each enclosing a narrow
band of fibrous tissue, the ventricular ligament, which is attached in
front to the angle of the thyroid cartilage immediately below the
attachment of the epiglottis, and behind to the antero-lateral surface
of the arytenoid cartilage, a short distance above the vocal process.
The lower border of this ligament, enclosed in mucous membrane, forms
a free crescentic margin, which constitutes the upper boundary of the
ventricle of the larynx.
The vestibular folds of the larynx play a greater role in keeping food
and drink out of the airway, breathing, and phonation (speech).
People who have had their epiglottis removed because of cancer do not
choke any more than when it was present.
They have a minimal role in normal phonation, but are often used to
produce deep sonorous tones in Tibetan chant and Tuvan throat
singing, as well as in musical screaming and the death growl
singing style used in various forms of metal.
They are lined with respiratory epithelium, while true vocal cords
have stratified squamous epithelium.