OT: A/B/Y box (sort of) for a headphone signal

[Chris S]

I'm looking for a schematic that will help me to do the following...

Take a stereo headphone signal and ...
1. send the left signal to a P.A at a line level
2. send the left and right to volume knobs which will mic them into one mono signal which will then go to a set of headphones.

Basically our drummer will be playing to a backing track on a CD the left side is the music and the right side is the click track, Hence the both need to go to headphones to the drummer but only the music needs to go to the mixing desk and at a line level.

If any one can give me any help with this or can refer me elsewhere that would be greatly appreciated!