The only thing you can get out of the one L1 would be MONO playback of a stereo source.

The Y adaptor SUMS the L and R channel to MONO.

This is desirable if, say, you have a stereo mp3 player or cd player that you want to hook up and amplify. You would want the people to hear both channels, but they wouldn't be able to hear true stereo coming from two different places. The sound would be both channels summed and coming out of one single source, thus it would actually be monophonic.


--Mac