"I’m happy to announce the release of a side-chain stereo expander based on the Audio Toolkit (yes, after a compressor it is time for an expander!). It is available on Windows and OS X (min. 10.8) in different formats. This stereo expander can work on two channels, left/right or middle/side, possibly in linked mode (only one set of parameters), and can be set up to mix the input signal with the compressed signal (serial/parallel compression). The side chain channels can be used to steer the gain stage (the same setup will be used, right/left or middle/side)."

ATKSideChainExpander is a stereo expander with a side chain stereo input. It can work in left/right or middle/side mode.