Is it better to have two matched subs or one

I am looking to complete my system.  I have one Salk 12 sub.  I am considering one more.  I enjoy classical.  Any thoughts
@salevick - How big is your listening room? 
Is it a dedicated room or shared living room, etc.? 
Do you have a budget in mind?

Hello salevick! For classical music, one is fine. For more ooomph in pipe organ music, percussion, etc. two is obviously better. The big reason for one per channel is home theater use. Crashes, alien approaches, creatures in the bushes sneaking up on you are better since a creature menacing from the left side should not have heavy footsteps thudding from the center of the room. Many TV documentaries and nature programs (BBC stuff in particular, PBS series also) have marvelous multi-channel soundtracks. I have five systems in the home and I have 9 subwoofers. Keep Smiling!
Wild thought....CS2 footers are a new concept in isolation and using them allows the system to work closer to spec. At least that is what many users report. We find that there is no need for subs when using full range speakers, and many have removed them. 

Not sure how this would work in surround or video applications.  But in my 2 channel setup there is now thunderous bass in movies and gunshots are pretty dramatic.  
aubreybobb - By full range do you mean essentially flat to 20 Hz or below?
I've read that additional subs help with the bass "in the room" but don't most people sit in one spot? And if that spot has sufficient bass from one sub, what do additional subs add? 

Regarding "stereo" bass, what MC says about the signal below 80hz makes sense, but what if the recording mix has different amounts of signal in L & R? Then, having just one might not be getting what's supposed to be coming through the other, right? Or maybe that's just a red herring, and songs are never mixed so asymmetrically.