Two subs provide a lot more flexibility. It may be easier to place two small subs (two 9" cubes in my 2-channel system) than one larger one. Two can help break up room nodes that cause standing waves. Improperly done two subs could be out of phase and cancel out the lowest notes, but properly done they will reinforce each other. For a good, coherent stereo image, the sub(s) should be in a consistent geometrical relationship with the listener and the main speakers. With a single sub you are often limited to a narrow range of ideal listening positions. Two subs make it easier to maintain a consistent geometry throughout the room.
A second sub also adds at least another 3 dB dynamic range.
I have two subs in both my 2-channel analog living room system and the 7.2 channel HT rig down in the family room. In each case there was a noticeable improvement in the clarity, imaging, and dynamic range when I added the second sub.