Indeed, the original Dolby Surround, and the first Prologic had a mono surround channel (even if you used two speakers). But I don't know how modern receivers handle only having one surround hooked up.

My Pioneer doesn't seem to have the option for that, only 6.1 a single rear with two surrounds.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris