2. Send the duplicate to a bus. In Pro Tools, I have sent it to bus 128 for example's sake. Open the compressor on the original track, enable its external side-chain and choose the same bus you sent it to as the sidechain input.
10 essential modern EQ techniques | MusicRadar
Pre- and Postfader
The Parallel Mix Trick | The Recording Revolution. Article: http://therecordingrevolution.com/2016/02/11/the-parallel-mix-trick/