My son recently mentioned a conversation that took place 10-15 years ago between him and one of his former Bible professors at YU with whom he had remained in touch for awhile after his graduation, but at this point it's been quite a few years since the last time they had any contact with each other. The topic of the conversation was the dageish in the tav of the feminine forms of the number two (shtayim, shtei, shteim esrei, etc.). This dageish obviously violates the usual rules of diqduq, i.e., a sh"va under the first letter of a word is "always" na, and a beged kefet letter following a sh"va na should not have a dageish.
My son reported that his former professor told him the following.
1. There is a mesorah that the sh"va under the shin is nax in these words, which explains the dageish at the expense of violating the usual rule about a sh"va under the first letter of a word.
2. It is a reasonable guess (but without support from any historical evidence) that these words once started with an alef, e.g., ishtayim and ishtay --- and the dageish in the tav remained even after the alef dropped out.
3. This mesorah does NOT extend to the the masculine forms of the same words (shnayim, shnei, shneim asar, etc.). Hence, according to this professor, the feminine forms should be pronounced with a sh"va nax, but the corresponding masculine forms should be pronounced with a sh"va na.
I asked my son if the professor had supplied any written sources for this mesorah, and he responded that he doesn't remember whether a source was supplied (and if it was supplied, he doesn't remember what it was).
BASED ON MY SENSE OF LOGIC, I HAVE A GREAT DEAL OF DIFFICULTY BELIEVING #3 ABOVE. It seems "obvious" to me that, e.g., the etymology of shtayim must be completely analogous to the etymology of shnayim, i.e., if shtayim came from ishtayim, then shnayim must have come from ishnaytim --- and the only difference would be that there's no residual dageish in the nun because nun is not a beged kefet letter.
IN CONTRAST, I HAVE NO TROUBLE BELIEVING #1 AND #2 ABOVE. It's true that a sh"va nax violates a rule about sh"va'im, but the alternative choice of a sh"va na violates a rule about d"geishim, and there's no reason for preferring one violation over the other.
IT FOLLOWS that I would be comfortable pronouncing both the masculine and feminine forms with a sh"va na or both with a sh"va nax, but I would be uncomfortable using a sh"va na only for the masculine forms. As a cute side point that's not relevant to the issue at hand, it occurs to me that using a sh"va nax for the masculine forms would have the appealing effect of creating a difference in pronunciation between shnei (two of) and sh"nei (years of).
Any thoughts? I'd love to see a source for this mesorah if it exists and to hear people's ideas about how to pronounce the sh"va.
Art