In a pure sukoon, we even don't pronounce the vowel. If the pure sukoon doesn't pronounce any vowel, how can we avoid the meeting of two sukoon?

Question
Asalaam Alaikum
I asked a question about the pure sukoon and rawm. Thanks for removing my doubt. But now it raises a new doubt. As you taught,a "pure sukoon" means to drop the vowel on a letter, and the "rawm" means to drop 2/3 of a vowel and to keep 1/3 of the sound. You also taught on the presented letter, such as the "m" of the letter "antum" it's prohibited to use rawm,but only pure sukoon is allowed, to avoid the meeting of two sukoon. But according to my understanding, Rawm=1/3 vowel while pure sukoon=0 vowel, at least we pronounce the vowel if it is rawm, thought shorter than an ordinary vowel. But about pure sukoon, we even don't pronounce the vowel. If the pure sukoon doesn't pronounce any vowel, how can we avoid the meeting of two sukoon? This question tortures me recently. So thanks very much for answering!

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
 is not allowed on a vowel that is a "presented" vowel, or a conditional one such in the case of a vowel added on a word because it ends in a sukoon and the first pronounced letter of the next word has a sukoon.  This vowel is not part of the original make up of the word and not normally there, instead is just a temporary one to be used to join the two words together and to avoid two saakin letters meeting between two words.  This is the reason only a pure sukoon is allowed when stopping on a word like this.  An example (taken from the lesson on the site is: .  The first word here is , and this word normally has a fixed sukoon on the .   In the example here though, the is follwed by , a  saakinah, the first pronounced letter of the next word when reading these two words together.  We then have two saakin letters meeting between two words, which is forbidden in the Arabic language.  To change the situation the acquires a dhammah, but only  for this situation.  It is a temporary dhammah and not part of the normal occurence of this word.  Therefore, it is not allowed to stop on this word with or .  The only allowed way to stop on this word is with a pure sukoon. 
The only time the letter word has a dhammah is in this type of case, so the normal case is that is does not have a dhammah on it. 
The pure sukoon here does not prevent the meeting of two saakin letters.  The temporary dhammah on the letter is to prevent the meeting of two saakin letter when reading this word with the next word.  The letter is not temporary, but the dhammah on it is. 
When stopping on the word there is no meeting of two saakin letters because of the stop. The next word is not pronounced when stopping on the word , so the two saakin letters between two words are not meeting.  Only when reading the two words together in continuum does the condition of two saakin letters occur, and then the acquires the dhammah to avoid the two saakin letters from meeting.
Insha' Allah this gives you more understanding of the situation.