Envelope Effecting Both channels

I’m having a little problem, I want to set a envelope for a stereo channel but instead of the envelope effecting both channels (L & R) evenly there being effected separately, how can I fix this?

Split the Stereo track - click on the little downward pointing black triangle in the Track Control Panel to the left of the track (easy when you know where it is - but not intuitive at all!) and select Split Stereo Track. Then you can manipulate the left and right channels independently. When finished click on the black triangle of the upper track and select Make Stereo Track.