2 subs in a small room?

A simplified answer: 

Any deep bass source will produce imperfect in-room frequency response as reflected long bass wavelengths collide in the room.  In some cases, the collisions are destructive (resulting in frequecy response dips) in other cases they'll reinforce (creating peaks).  The good news is, with subs you can locate the unit to minimize the problem.  With full range speakers, the bass generation goes along with kids and highs - to a spot where overall response sound best.

Now, think about potentially  "fixing" the bass dips, for a second.  The frequencies at which the dips occur are primarily a function of the distance from the sub to the nearest wall (quarter wave effects).  Place the first sub wherever it shows smoothest response.  It won't be perfect, but it's probably better than you'll get from full range speakers out in the room.

Now, add a second sub.  By properly positioning the second sub (probably significantly further from or significantly closer to the nearest wall than the first sub)  you can generate a completely different frequency profile of response irregularities.  The trick is to position the second sub at a place in which its irregularities will offset the first sub's irregularities.

If you can manage to get it even close to right, the summed response from the  two subs is smoother than the response from the single sub.  Add a properly positioned third sub and it will be smoother still.  A'goner Duke L. sells the four unit swarm subwoofer system which is designed around this idea.  This is the primary benefit of multiple subs, IMO.

As a second (completely unrelated) benefit, you will also tend to reduce distortion when you add subs.  To generate a given SPL from any sub, you'll require it's driver to travel a given distance (excursion).  Add a second sub and the requisite excursion for each of the two drivers to generate the same total SPL is reduced.  Distortion in subs increases (usually dramatically at some SPL) with driver excursion, so you get improvement there, too.

Bottom line:. If you have the budget and space to put a second sub where it belongs, it's usually a good idea to do so.

The room will load up easier being smaller, two subs is a great idea, big room small room dont hesitate.

Thank you for the great response martykl.  So how hard is it to integrate a second sub?  I have read from others that setting up a second sub is difficult and an EQ is a must.  

In my room, 2 subs actually produced worse frequency response than 1. Getting rid of the extra sub proved costly...

Tbooe,

Doing it by ear will usually take time.  It's a bit of a PITA, but I've done it successfully several times.  Unlike Coli, I've never failed to achieve smoother FR with a second sub.  If you use an SPL meter or a RTA, it's much easier.  

These days, I use Audyssey.  It's certainly not an audiophile approved method, but IME it's a 100% foolproof, no-sweat method of smoothly integrating one or two subs into a system.