Watch the Octaves online guitar lesson by Annie Raines from Blues Harmonica Blueprint

This is another style of tongue-blocking commonly called "octave" tongue-blocking. I call it "split style". This time you will use the tip of your tongue. You’re going to play a 1-2-3-4 blow chord, and gently set the tip of your tongue right on the comb of the harp, centered on the little post between holes 2 and 3. Now let your tongue protrude enough to fill in the 2 and 3 holes, blocking them and letting the air flow on both sides to play 1 and 4. You should hear both the high and the low note, and an in-tune octave has a distinctive, "clean" sound.

One of my students recommended using scotch tape to cover the 2 holes in the middle in order to learn to recognize the sound of the octave - some people also place their thumb there - anything you want to try, as long as no one gets hurt.