T99, welcome. The primary advantage of two subs is(as Randy pointed out)that if they're properly positioned(i.e., the mid-points of opposite side walls or front and back walls or diagonally opposite corners)they can help to minimize the uneven effect of room modes on bass response and result in more overall smoothness.
The two subs when widely separated increase the sound level by 3dB compared to one of the same. There's no direct effect on extending the bass, except that a given frequency will be, as mentioned, 3dB louder. The relative ability of the bigger single sub as compared to the two subs would determine the overall bass level produced. If the single sub was greatly superior to each of the twin subs, the net effect would be that it would still provide greater extension and maximum loudness, although not the overall smoothness.
