Now,
this is the first time I have ever heard about any 2:1 rule. I cannot fathom how in any way that would help in setting the distance between the mics making up an AB stereo pair.
In my mind I think we could scratch that 2:1 rule forever.
When recording symphone orchestras I very often use two omnis. They are placed far enough into the room to get the best balance between direct and ambient sound, to capture the right amount of room. The distance depends a lot on the acoustics of the room. Sometimes one omni pair will not be the best solution.
The distance between the mics is set to get as good stereo separation as needed. Too far apart and there will be a large hole in the middle of the stereo picture, too close and there will not be enough separation. In my experience the distance between the two mics goes between about 1 foot up to maybe 8. In my recordings I generally end up at about 1.5 foot.
One different technique usable on symfony orchestras is to use three omnis, rather closer to the stage than with an AB pair. One goes in the middle, the two others flank it about 1/3 and 2/3 of the width. The mics are panned left, mid, right and the volumes set to a pleasant mix.
This is of course not very relevant to stealth micing of rock, where most of the sound comes out of the PA speaker stacks and the room ambience is mostly seen as a nuisance. I think you could use omnis to good effect in that environment as well. I would put the two mics on a stereo bar about 2 foot apart and hoist it high up in the air, using a stand at least 12 foot high, preferably more, to get away from the audience chatter.
Gunnar
