♫ "Dude, he's like, whistling twice!" ♫ "Wow, how do you do that?" Well, ya'see, it's kinda hard to explain...
Dual whistle (♫): A whistle comprised of two separate notes, the whistler having the ability to change either at any given time without affecting the other. For humans, accomplishing this is a matter of concentration and tongue dexterity. One way to whistle two separate notes simultaneously is by separating the mouth into two air passages, one for each note, then blowing air through them and adjusting the positioning of the tongue appropriately. The separation must be complete at a minimum of one point, otherwise the two notes revert to one. A picture would be interesting, but not entirely helpful, as a person exercising this particular whistle looks weird. With a fair amount of thought, and a fairly equal amount of experimenting, this whistle can be achieved in a relatively short amount of time, on the order of 2 to 3 hours. When achieved, it is a rather warbling sound, and tends to come very close to the natural sounds of some birds.
Yes, I can whistle two separate notes at the same time. Now that I've figured it out, it isn't all that difficult (to me), but I have a hard time explaining how to do it.
I think the best way to describe how to whistle two notes at the same time is to think about the way Donald Duck talks. Have you ever tried to make a Donald Duck noise? It's often done using the space between the teeth and the cheek on one side of the mouth or the other. Now try talking like Donald Duck on *both* sides of your mouth at the same time. Stop and relax. Make air passages between your upper set of teeth and your tongue (on the "inside" of your teeth), and blow air through those passages slowly. Experiment around until you get one side (or the other) to make a whistling sound, then work on getting both at the same time.