In the October 2011 issue of Stereophile, John Atkinson reviews a speaker that uses a second rear-firing woofer, the Vivid Audio B1.
I assume Vivid's primary reason for this approach was to increase the bass and lower midrange output without spoiling the elegant appearance of the speaker. The front-firing woofer goes up to 1 kHz or so, and the rear-firing woofer rolls off gently above 200 Hz or so, such that it's still making a significant contribution up around 500 Hz, and is probably providing baffle-step compensation.
The frequency response curves are quite interesting. We see the wrap-around notch centered on about 280 Hz in the composite quasi-anechoic curve, and we also see that the wrap-around notch is mostly gone from the in-room curve taken in Atkinson's room, and
completely gone from the in-room curve taken in John Marks' room.
The wrap-around notch occurs at the frequency where the wrap-around energy from the rear-firing woofer arrives at the listening position one-half wavelength behind the first-arrival sound from the front woofer, and it is pretty much swamped out by the in-room response.
Here's an article I wrote about a type of bipolar configuration (which I use in several of my speakers) back in the inaugural issue of HiFi Zine:
http://www.hifizine.com/2010/06/the-controlled-pattern-offset-bipole-loudspeaker/
