Two reasons that I can think of are the point and slope of the cross over - if it is too high or two shallow some signals (in stereo) will become a mono source from a 3rd location. This will muddy up the clarity of the image in that frequency range. Also two subs don't have to work as hard as one to produce the same volume. You can use a sub with a smaller and faster driver and get the same effect, but a much cleaner one.
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