Some time ago bibliofile posted Punctuation question: two-dot ellipsis. I was at work when I read the post and answered as well as I could under the circumstances. I finally remembered her question a couple of days ago when I was at home and could look through some of my reference books.
Of course, first I tried H.W. Fowler's Modern English Usage, revised and edited by Sir Ernest Gowers. I was delighted to see a two-page entry on the subject; however, as I read it I became confused and ultimately disappointed, as it addressed the thing itself* rather than the typographic representation of its occurrence. Oh well.
I poked around a bit more, and finally came up with a pretty thorough discussion of typographic ellipses from a copy of the U.S. Government Printing Office Style Manual; Revised Edition, January 1973. Since HTML is not particularly good at preserving precise typographic layout, I reproduced the entry in pdf form, which you can find here: Ellipses.pdf
However, I still haven't found any reference which mentions the "two-dot ellipsis" bibliofile remembers from her school years….
*The omission of a word or phrase necessary for a complete syntactical construction but not necessary for understanding.