I accidentally stumbled upon singing two notes simultaneously whilst in the shower once.

I am now able to sing a single note, and by expanding and contracting my mouth cavity in a controlled way, I am able to create a standing wave in my mouth that produces partials (harmonics) just as you would find on a guitar if you gently touch the string halfway along, or a third of the way, or a quarter etc etc.

The more I meddle with it the better I get, and I can choose which partial (harmonic) I want pretty much at will now.

I recommend it!