I can't answer your question directly as I only have one x power running in stereo. However I have heard two x powers running in mono, and know that lots of people over on the unofficial cyrus forum love that combination.
Personally in my size room at the volume levels I listen at, one is enough. What speakers do you have, what size is the room, how far away from the speakers do you sit, and how loud do you like to listen?
if it was me, I would probable buy a second on off the cyrus unofficial forum classifieds (or eBay) as you can pick them up for £500 or so, and give it a go. At least then you can make up your own mind.
The only thing is for the same net cost, you could buy a pair of mono x's for less than £1000 (from the same places) and sell your current x power. Better again? Maybe, only you can say!