The 3.5mm Stereo Female to 3.5mm Mono Male Adapter from CablesToGo can function in either signal direction. It can take a mono signal that would normally play out of only one headphone, and copy it to both. It can also take a stereo signal and sum it to mono.
Note: This adapter WILL NOT produce stereo sound.
In the Box
C2G 3.5mm Stereo Female to 3.5mm Mono Male Adapter