The rule is as stated by others in the comments, that the Yud is part of the ending. As such, if breaking it up into parts, it would be most correct to portray the visualization of it as Ya-DA-Yim, as described by @Double AA.
However, I believe it is incorrect to deliberately pronounce it that way - and by that I mean pronouncing it with deliberate emphasis on each of the syllables.
There should, in my opinion, be a diphthong when it is pronounced audibly, much like this:
My view on this is shaped by some limited experience studying Semitic languages. I am far from an expert, though I would play one on TV, given the chance, but I believe the Hebrew language, much like its sister languages, has a natural fluidity when pronounced, unlike Germanic languages (like English), which have more rigid stops. To be fair, Hebrew does have stops, and the Ba'alei Mesorah have encoded for us certain forced stops in certain syllables, but I believe this is proof of the natural inclination of Hebrew to follow this pattern: that is, the fact that we have certain places where rigidity is enforced shows that the language has a natural fluidity to it.
I cannot speak for the author of the book you have studied, nor for other (more professional) opinions on the matter, but in most cases, especially as a student, when I have asked more advanced experts (both academic and religious, most times overlapping) on similar subjects, I've been pushed in the direction of allowing fluidity, rather than rigidity, in the pronunciation of Hebrew.