According to Polk Audio web site....
"If your seating area adjoins the rear wall, or if you cannot place speakers on the side wall, you'll have to position the speakers on the rear wall (Fig. X). In this case, bi-directional (bi- or di-pole) speakers are recommended, since they will produce the most diffuse effect. If you use front-firing speakers, do not aim them at the audience as one would a front speaker, but point them straight forward so the sound projects past the audience (figure below). Another option is to mount in-wall speakers in the ceiling, aiming them downward at the listening area."
They seem to suggest ceiling surroud left and right speaker rather than mounting on rear wall.
sjm