Brian, when the modes such as DPLII or Neo:6 have only one back speaker(i.e., 6.1)it's sent part of each side surround speaker sound. Sound that's equal or near equal in the side surround channels is intended to be imaged in or near the back center and the back center speaker can do this.
Two back surround speakers, however, can carry varying amounts of sound from left back to center back to right back and at times give a back "stereo" effect. Again, the separation that could be had here would be enough to realize that effect.
