A combination of omnidirectional microphones can be very effective for stereo recording.
Spacing two omni mics a few feet apart from each other is a good approach if they are rather distant from the sound source. This takes advantage of the wide and smooth frequency response that these mics tend to exhibit (especially the higher quality ones).
Start by aiming one mic towards the left side of the sound source and the other towards the right. Listen both in stereo and mono. Be aware of any phase cancellation in mono, and change mic separation, aiming, and distance to sound source (if possible) to get a good stereo sound field that also works in mono.
This mic technique tends to pick up both the room ambience along with the performance. Listen for the balance of the two and make adjustments as needed.
The microphones should be the same type and have the same frequency and phase response as well as sensitivity.
Another approach, called the Decca Tree, uses three omni mics--left, right and center. The left and right mics are spaced about six feet apart and the center one is located between the left and right but placed forward by about four to five feet. This configuration somewhat resembles a letter "T" and has found success in orchestral recordings.
Place or hang this setup in front of and above the sound source. Six feet is a good starting distance, but the final placement will depend on the size of the room, room acoustics and the size of the performance area.
With three mics to form the stereo image, mixing is necessary. Pan the left mic to the left channel, the right mic to the right channel, and the center mic to both channels. Set the levels for the left and right mic to the nominal operating level, and the center mic at a lower level. Start with the center mic 5 or 6 dB down from the level for the left and right mics and balance the level so that it neither dominates the left and right mics nor disappears altogether.
Like the spaced omnis, the Decca Tree will pick up room ambience, which could be a good or bad thing depending upon the room.
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