A-B Stereo is a recording technique. For this you have to use two identical omnidirectional microphones placed some distance apart. A-B stereo microphone technique is often used when the sound source is wide and/or when the distance from the microphones to the source is significant or become somehow relevant in that process of recording. Omnidirectional microphones are used to maintain consistent low-frequency response regardless of distance. A little attention to microphones placement will result in a wide stereo field with a good balance of room ambience and direct source signal.