It means pan the nat sound to the left (channel 1) and pan the music to the right (channel 2). I think you can do this in AE, but I’m sure whatever you use to go to tape (like FCP or Premiere) can do it also.
This is so they can use one audio track without using the other, or they can mix the levels however they wish.
Don’t worry about having it sound bad having music from one side and nat sound from the other – they will (or should) mono mix it out on their end