How to use a stereo microphone
Simply put, a stereo microphone is two regular studio microphones in a single unit.
Really, it's that simple! They are arranged in various ways to achieve different results or sounds, but the unit is the same - a stereo mic is two diaphragms or capsules on one microphone body.
They are distinct from a matched stereo pair of mics. A lot of mics out there will be available in a set, advertised as a stereo pair, but it is not a stereo microphone. A stereo pair is two separate mics, a stereo mic is one integrated mic.
Take this Rode NT4. It has a barrel like thing that most mics have, but at the end there are two capsules or diaphragms, pointed 90 degrees from each other. You need a special cable to go from the mic to regular mic cables (that is supplied, yippee!), but once you do that you have a genuine stereo track - two mono tracks.
The NT4 microphone has its capsules placed in a fixed XY stereo position. They are cardioid, and the setup works quite well. For more info on the NT4, either wait for my upcoming review on it or check it out at zZounds.com.
Stereo mics usually are in an XY or Blumlein configuration, because of physical location. If you want to do a Decca Tree setup, you can't really have one mic spanning 3 or more feet - it's just not feasible. See the stereo recording page to learn more about the stereo recording configurations and how they work with mics.
Another stereo mic to check out is the Royer SF12, a ribbon stereo microphone.I've heard good things about ribbon mics, and really want to try one sometime.
There you have it. A stereo microphone is just an upgraded mono microphone. Same as if you plugged two other identical mics in. Do you have any questions? Let me know!