Thomas O. Lambdin calls out the words שְנַיִם and שְתַּיִם as exceptions to "shewa as the initial vowel in a word is always vocalized," i.e. it is "shtayim" and "shnayim". That's a secular expert's view.
I recently moved, and the Rabbi at my new shul says "shi-tayim" and "shi-nayim." I asked him about it, citing in particular that in שְתַּיִם the tav would not have a dagesh if the shewa were vocalized. He said he has a source, but I haven't seen it.
What is the halachically correct pronunciation: "shtayim" or "shitayim"?