side-chaining - ducking in Reaper

Following the method given at AskAudio:

1. Drag and drop from the io button of the track which is going to be the controlling track, to the track which is the one which will be affected by the compressor.
2. Choose drop-down and change the destination of the receive to 3-4 instead of the default 1-2.
   
   
   
   
   
3. Choose the Detector Input on the compressor to be Auxiliary Input L+R and other settings as per your choice.