Aharon from Monsey wrote:
Why is the name of Hashem written in the siddur as two Yuds? In my siddur - siddur "Tefillat Kol Peh" - it is written as two yuds everywhere that I looked, with the exception of Shirat Hayam (the song at the sea). Why is this so? I suspect that there is more to it than merely an editor's whim.
I'm afraid your suspicions are groundless. The phenomenon is purely a function of your siddur's editor and printer. The double yud is not one of the names of Hashem and has no kedusha (sanctity). It's simply a substitute for the name of Hashem and it appears randomly in various versions of the siddur. The siddur I use (Kavanat Hashem) has the name of Hashem written out fully all the time.
Essentially, the reason for using a substitute is out of respect. The name of Hashem has kedushah and must not be erased or abused. Since two yuds isn't a name of Hashem, it was substituted so that if the siddur wasn't treated properly it would be less serious. Especially today, with the availability of inexpensive printed siddurim, siddurim aren't treated with the same care and respect with which they historically were treated.
I called Eshkol Publishers, creators of the siddur 'Tefillat Kol Peh.' I asked them why in some places the name of Hashem is written as two yuds and why in other places it's written out fully. They told me that when they put together the siddur they copied (with permission) from various older siddurim by means of offset. Whichever way the name of Hashem appeared in the old siddur, the new siddur had the same.
But why two yuds? I heard the following reason from Rabbi Aharon Feldman, shlita: The Name of Hashem is written one way and pronounced a totally different way. It's written with a 'yud', a 'heh', a 'vav' and another 'heh'. However, we pronounce it as if it were spelled 'aleph' 'dalet' 'nun' 'yud'. As you can see, the letter 'yud' appears twice, once in the beginning of the written Name and once in the Name as pronounced. Hence, two yuds.
- Shulchan Aruch, Yoreh Deah 276:1