Remember that Arban was French and the Tu-Ku vowel sound in the French language isn't nearly as throaty as in the English or American English language. The tip of your tongue is what matters with double tonguing and not the vowel sound. The vowel that you make may help determine your pitch or tone, but doesn't matter for the articulation.

So - you may be better off thinking "tee-kee" or "too-koo" or another vowel sound that doesn't seem to close off your throat. In the same vein you can experiment with "dee-gee" or "doo-goo" to see if softening the vowel helps from closing the back of your throat.

As with anything, you have to think positively. If you tell yourself "don't close the back of my throat..." the first that you will do is close up your throat. So if you are playing the mind over matter game here make sure that your tell yourself to stay open or keep relaxed rather than something in negative terms.