It's called the "3 to 1 Principle". From
The Recording Engineer's Handbook by Bobby Owsinski (p. 77-78):
Quote:
The 3 to 1 principle is pretty important when considering any multi-mic setups because, if you observe the rule, you can stop any phase problems before they start. Simply put, the 3 to 1 principle states that in order to maintain phase integrity between microphones, for every unit of distance between the mic and its source, the distance between any other mics should be at least three times that distance. For instance, if a pair of microphones is placed over the soundboard of a piano at a distance of one foot, the separation between the two mics should be at least 3 feet. If the distance from the source is 2 feet, the distance between mics should be at least 6 feet.
The principle is not a hard and fast rule, but it certainly is a good guideline for eliminating phase problems.
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(This is a great book, btw - I highly recommend it! It also includes a good section of interviews with different professional recording engineers.)
There are other stereo recording techniques that can be used for recording of a source (say, a choir) in a stereo field. These would be methods like X/Y, M-S, or near coincident Pair. I imagine there would be a lot of information available about these that you could find with a google search. I've personally used the near-coincident pair method on a smallish choir and had great results. In that method, you put two cardoid-type mics pointing about 110 degrees apart, with the capsules about 7 inches apart horizontally. I didn't quite get the angle correct, but it still turned out.