Pronounce the sound "tu" several times and notice how the tip of the tongue hits inside the mouth between the top of the teeth and the roof of the mouth. This technique allows you to single-tongue.
Pronounce the sound "ku" and notice how air quickly expels from your mouth. Pronounce the word quickly and precisely. This sound will create the second articulation in a good double-tongue technique.
Play a note on your horn and mimic the sound "ku" several times in a row. Once you get a clean articulation using only your air add the first syllable "tu." Imitate the motions of both of these words while you play so that you are pronouncing "tu-ku."
Increase your speed gradually and say the phrase "tu-ku, tu-ku, tu-ku, tu-ku" over the course of four beats. Practice until you can perform this action comfortably.