If Metal Man's suggested hardware doesn't work, you could use:
1) A 3.5mm headphone splitter (intended to use two headphones on one jack, chained to:
2) Two 3.5mm to RCA stereo adapters, resulting in two Left ends with sound and two Right ends without, chained to:
3) An RCA to 3.5mm adapter, taking the two Left RCA's as input.
Stupid, and will result in the volume being halved, but it would absolutely work.