Monk, welcome. As both Ken's replies and the Crutchfield article indicate, what you propose is feasible. Your Onkyo 260 has line level Zone 2 outputs to connect an external amplifier with the usual coaxial cables terminated with RCA plugs. Either the same or different source material can be played in Zone 2 as is played in the main room.
In your case "Zone 2" is actually two separate rooms, both driven by the stereo receiver. The A/B switch on the stereo receiver would determine which(or both)Zone 2 room would be playing. Zone 2 would be turned on or off by the 260, or simply by manually turning the stereo receiver on or off.
