I never really attempted to do this before.
I'm quite familiar with how to do this with hardware but not software...
Until now...
There are 2 software plugins that know of that do this. db audioware Compressor (directx plug-in) and the TC Works Native Bundle (compressor and sidechainer VSTi plugins)
First for this example lets have 2 channels, one with the kick drum, one with the bass line that I want to compress...
put the TC Works Sidechainer VSTi plugin on the kick drum channel as the insert, or with the db audioware, insert the compressor plugin but keep it at 0 (zero) settings and set the "send" to "A".
Next, insert the TC Works Compressor VSTi on the bass line channel and set the "Key Input" to #1. With the db audioware, insert the compressor and set the receive to "A" and there you go, the compressors are listening to the signal from the kick and not the bass line. Have fun ducking!!!
Though this may not be new to some of you, I found this tricky to play around with in the software world and thought it would be good to share.
If there are any other software compressors that can do this, please fill us in.
-Patrick