Subwoofers are a great addition to a system and I personally recommend running multiple subwoofers whenever possible. Below are a few reflections and benefits of running multiple subwoofers in a system:
Aesthetics can play a large role
Two or more smaller subwoofers may very well outperform a single larger unit. Also when it comes to the visual impact of the room and decor, multiple smaller units may be preferred to a single bulky subwoofer.
Get Rid of Room Mode Problems
Any given room is a confined space with walls, floor and ceiling. Sound waves will be reflected from these surfaces and bounce back and forth. At any given point in the room certain frequencies will become much louder due to constructive interference of reflected waves, and certain frequencies will become much weaker due to destructive interference. This is a physical phenomenon caused by placing a sound source in a finite room with partially reflective surfaces. It has absolutely nothing to do with the size, quality or type of subwoofer, this is physics at play.
We can employ the approach of room treatment to reduce the reflections, but for bass frequencies this is very expensive and intrusive due to the sheer bulk of absorbents necessary to have any significant effect.
We can also choose to equalize the signal however with a single bass source this is less effective than most people realise. We can suppress the peaks, but boosting the nulls is largely ineffective since there is still a null regardless of the magnitude of the excitation. In any case such equalization will be targeted at a single listening position in the room and will usually result in other listening positions being negatively effected.
With two or more carefully placed subwoofers we have immediately reduced the magnitudes of such peaks and nulls by a huge margin, and also the bandwidth of peaks and nulls will be significantly reduced resulting in a much smoother frequency response at any listening position. Equalization is also much more effective with multiple bass sources, and the built in equalization filtering technologies of many A/V processors will do a much better job with multiple sources compared to a single source.
Two or more subwoofers will naturally outperform a single subwoofer of the same kind. Very often two or more smaller subwoofers will outperform a single larger unit.
With more available headroom the bass quality will increase and there will be a sense of effortless bass due to reduced compression, lower distortion and greater dynamic range.
Leaving the Bass in Stereo
Using dual subwoofers in a dedicated 2-channel system will preserve bass cues in stereo, and further help expand the soundstage and improve imaging. This can of course be particularly important when smaller speakers with limited bass capabilities are being used.
Let Go Of Localised Bass
Even the most carefully integrated subwoofer may sometimes still suffer from the listener being able to hear that the deepest octaves are emitting from a single source in the room. Going from one to two considerately placed subwoofers will immediately get rid of this potential artifact, even if both subwoofers are fed with a mono signal as is usually the case with an A/V processor with LFE output(s).
Once you have experienced the benefits of multiple subwoofers you will never want to go back!