Dispersion can be defined as sound waves passing through a given material and then into its various frequencies. The speed at which sound waves separate into various frequencies can have an impact on sound quality. Single woofers generally are noted for creating unwanted, narrow "sweet spots." Dual woofers do not stop this problem from occurring but they do broaden the focus of sweet spots. This equates to more equal dispersion of sound.
Dual woofers gain a significant advantage in terms of surface area. Though the use of small woofers might indicate poorer frequency response, the fact that dual woofers use two cones means that problems associated with low-end volume are negligible. Even in the case where two smaller cones do not match the diameter of a single woofer cone, the use of two cones with speakers of sufficient quality can enhance frequency response.
The use of two smaller woofers can carry a distinct advantage in terms of tightness. This increased tightness of sound translates to improved frequency response. It is important to mention however, that the quality of any speaker is only as good as its design. A low-end single woofer, for example, can not compete with a high-end pair of woofers. With this in mind, using two quality small woofers will create sound that is much more defined than a loose single woofer.
The biggest enemy of dual woofers would be their inability to respond to super low frequencies -- anything near or below 20 hertz, the general limits of human hearing. Coupled with appropriate crossover levels to a single subwoofer, however, and this common problem can be dealt with so low-end capabilities are enhanced.