I've got a schematic for such a circuit in a blue book titled something like Experiments in Audio.
L-R - R-L circuit for extra speakers
In the past I have wired a pair of extra speakers in series across the positive outputs of the terminals of a seperate amp to give the L-R/R-L signal to these speakers. I was wondering if anyone knows of a cicuit similar to the Carver/Sunfire "seven axis" feature where the side left speaker gets the L-R and the side right speaker gets the R-L program info.Thanks.