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.

Tbooe 

If my understanding of Dirac is correct, it should be a breeze to integrate a pair of subs.  If you're not using it yet, you may find that your existing, single sub set-up sounds much improved.

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.

Tbooe,

I find that there is usually a major improvement in the upper bass and much of the mid-range (say the two octaves below middle A and a fair bit of the octave above) when you get the subs integrated properly.  "Properly", in this case, includes smoothing frequency response thru the x-over point.  That smoothing can be done with more subs, EQ, or room correction (or any combo of the above).

I can't specify exactly why, but I'd speculate that it's probably a matter of cleaning up hash and overhang around/above the crossover point.  If not addressed, I believe it may have the impact of muddying response in the area described above.  Whatever the reason, the effect is usually very noticeable.  If you run Dirac, just a-b (single sub or a pair) with correction engaged and then off-line.  My guess is that you'll immediately understand what I'm talking about.

Tbooe,

I'm not sure I'd want to slice my comment that fine.  A single subwoofer system that is smoothed with the addition of a second sub, EQ and/or room correction will IME usually produce a sense of cleaner mids than it did prior to such smoothing.  As to the conditions required at any specific moment to hear that, I honestly can't say that I've paid enough attention to specify.  I can say that the effect has always been pretty evident (and pretty quickly evident) in the systems I've adjusted in this fashion.