Some information about using multiple subs:
Generally, multiple subs do aid in smoother bass response, of course the important aspect of getting good bass is finding the best placement for the sub based on the listening location. Adding 1 or more subs could also aid in increase in total output by almost 3db (100% increase in spl) , which is sometimes very desirable for low frequency dynamics involved in Home theater soundtracks.
Now, if you are using equalization too get a flat response from your 1 sub then it is mostly tuned for a single listening location. Other listening location may not be able to enjoy the same flat response. Current research (by Floyd E.Toole at Harman International) demonstrates that by having 2 or 4 subs one could achieve flat response at all listening locations. However, if the user is only concerned about one listening location then 1 sub (with equalization) will do just as well.