The XY pattern is used with directional microphones (usually cardioid) pointed in different directions. Cardioid microphones reject signal coming from different directions. Therefore, even if the mikes somehow occupied the same space, they would be hearing different things -- as long as they are pointed in different directions. Your statement that "the two signals obtained are equal in amplitude" is therefore incorrect: when instruments are miked using the XY pattern, there are differences in amplitude if the sound source (including reverberant reflections of sound from surfaces in the room) are coming "off axis" of the stereo pair.

The XY pattern is used with directional microphones (usually cardioid) pointed in different directions. Cardioid microphones reject signal coming from different directions. Therefore, even if the mics somehow occupied the same space, they would still be hearing different things (as long as they are pointed in different directions). 

Your statement that "the two signals obtained are equal in amplitude" is therefore incorrect: when instruments are mic'ed using the XY pattern, there are differences in amplitude if the sound source (including reverberant reflections of sound from surfaces in the room) are coming "off axis" of the stereo pair.