Why does one need two Paradigm X-overs?

Chazzbo,
The x-30,used as a single unit, "sums" the low end for a mono signal only. So if you want "stereo" subs, you have to use two x-30's. One for the left channel sub and one for the right.
Good luck and happy listening!

Hi Chazzbo:

The Paradigm X-30 allows control of two subwoofers, but you can adjust phase for only one of them. If you want to (or have to) place two subs at different distances from your main speakers, you may want to use two of the X-30 crossovers to allow phase adjustment for each sub, and "fine tune" the subs for the best effect in your room.

This way, the blend with the main speakers can be made as seamlessly as possible, and the bass response can be smoothed, with minimal peaks and dips. Two subs can really help to eliminate standing waves in the room, by radiating bass energy from two discrete sources, and giving you more flexibility in placing them for the best effect.