The way we do most of ours is with an Auto A/B Switch. Russound makes them, Sonance makes them, Niles.... They have a master RCA input and a secondary. When it senses audio on the secondary it will switch over for a preset time and then switch back. Makes for a clean install. Jobsite makes ones that has a built-in amplifier if you want to just have one main source plus the secondary.
Chris