To create a stereo image with two omni mics, you must have them in different positions in order to use the timing differences to make a stereo image. But remember, we already chosen the one perfect spot for the one omni mic, so it follows that if you want to use the AB stereo technique, you will have to put the second mic in a position that is not as nice as the first position.
This is a compromise you need to be aware of, which also affects the mono-compatibility. Using directional mics in a co-incidental arrangement uses level differences rather than timing difference to create the stereo technique. Of course, using cardiod mics, we now have the proximity effect to deal with!
So if you choose to record with the XY technique, you must be aware of this compromise, but you win on mono compatibility. As always, awareness of the pros and cons is imperative to your decision making as a recording engineer.