There will be some tipping point at which the probability of TWENTYTWO is higher than the two words TWENTY and TWO. Make sure you’re catching all of the places in which TWENTYTWO is referenced in the file — it will appear in the 1-grams, 2-grams and 3-grams. If “TWENTYTWO” never matches that could be due to something wrong with its phonetic dictionary (.dic) entry.
Silly question — why can’t you make use of the hypothesis “TWENTY TWO” in order to call the method you would call for the hypothesis “TWENTYTWO”?