Ah, see Trev updated the o.p to mention the classic XY approach.
I will explain this (as Trev forgot) because it is really useful for one big reason. It works incredibly well when you have to resort to mono (ie checking radio compatibility etc).
In a nutshell, it basically involves using two cardioid or hyper-cardioid microphones. Ideally, you want them as close together as possible, but at an angle of between 90- 135 degrees. Reason being the shape of the polar pattern. Setting the mics up this way will still give you a stereo image when you pan the inputs apart, because they are effectively picking up in opposite dierections. The stereo image is created by the decay of volume, rather then in the more conventional A/B mic method that generates a stereo image by recording the delay of the signal source between the two mics.
There are a couple of things you should know about the Blum/XY method. Ideally, for X/Y you should have a matching pair of mics (in terms of their frequency and polar response). If the mismatch is significant, then at best you will get unavoidable skewing of your stereo image. At worst, the technique won't work at all because the imbalance will be overly noticeable.
The Blum technique is often confused with X/Y, but works using a figure of 8 response and a cardioid, and basically sets up for M/S recording, rather then L/R. The fo8 mic is used to record the side information, whilst the cardioid focuses on the front/middle gap left by the fo8.
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