When I was first learning to record, the usual recipe for setting up a pair of omni microphones was to take the full stage width, then place the mikes at the 1/3 and 2/3 points along the front of it. That often meant that the mikes would be 10 feet apart or more. That creates a stereo image with a "hole in the middle" and "swimmy" localization in general. People might put up with those problems, or not perceive them; not everyone cares very much about localization.
Meanwhile over in Europe, the typical usage of omnis was quite different: They were often placed with anywhere from one meter to half a meter between the mikes. Any Americans who saw a setup like that might assume that there would be no "stereo separation" in the resulting recording, but such was not the case.
The thing is, if distance between your two omnis is only a few inches or less, the resulting recording will be essentially mono in the bass. And that means that you're missing one of the main pleasures that an "A/B" recording can offer, which is spaciousness. It comes mainly from low-frequency energy that isn't strongly "correlated" (highly similar from moment to moment) between the two channels. You will be able to localize to a surprising extent, especially if you listen through headphones, but the feeling of immersion in a live space will be absent.
Nowadays there are much better options available: small, single-diaphragm condenser microphones with patterns between omni and cardioid, and very smooth all-around response. These microphones can make beautiful, spacious stereo recordings (like omnis) with stable stereo imaging (like directional microphones) in a way that is simply not possible with omnis or ordinary directional microphones--and they can do this with spacings even less than the typical "small A/B" approach. These are still mostly higher-end microphones (Schoeps, Neumann, Microtech Gefell) but I think in time there will be good-sounding, lower-cost alternatives as well.
P.S.: If you absolutely must use a pair of omnis in this way, consider getting sphere attachments for them, which will emphasize their midrange directivity. See for example
http://schoeps.de/en/products/ka40 (please visit the "graphics" page to see the effect).