What's your goal? Are you looking for louder output, or lower extension, or just smoother bass response. If the latter, what are the main issues with the current subwoofer response? Are there any bad room modes or deep cancellation nulls that you hope to address?
In my experience, two subs are definitely better than one when it comes to producing flatter bass response. There are placement tricks you can use to cancel an axial mode, fill in a null, etc. I think that in most rooms, the benefits from adding another sub outweigh the concerns over matching. But if you're already getting pretty flat response from your current sub, then it might be better to just replace it with a more powerful model that can reach lower.
HT: Dynaudio C2, Contour S CX, 2x BM14S, Aperion surrounds, Simaudio Titan, Marantz AV8801, Oppo 103, Linn Majik DS, and a Pioneer Kuro
Stereo: Dynaudio Focus 160, Simaudio W-5 LE or Luxman M-600A, Linn Akurate DSM, 2x Rythmik F12G
Other interests: motorcycling, skiing, being active