XY has no meaning with omnis - you would get basically the same sound from both mics. Because the two mics are never exactly coincident nor have identical characteristics, there would be phase isssues at high frequencies if you tried to mix the two signals or attempted to make a stereo image by panning.
This contrasts with spaced omnis in A-B configuration, where the arrival time difference between the mics creates the stereo soundfield.
I would agree with Boswell in general; omnis are not useful when doing X/Y stereo recordings. But I've used two DPA 4003 (omnidirectional 130V mics) with acoustic pressure equalizers (emulating a flush-sphere mounted diaphragm) in a vertical X/Y setup on acoustic guitar when I recorded a singer/songwriter demo.
I feel that it gave me a great mono-compatibility while providing better sense of depth than one mic.