FreewareVoxengo MSED

MSED is a professional audio encoder/decoder AU and VST plugin for
mid/side processing which is able to encode (split) the incoming stereo
signal into two components: mid/side pair, and vice versa: decode
mid/side signal pair into stereo signal.  MSED is also able to work in
the “inline” mode with the ability to adjust mid and side channels’ gain
without the need of using two plug-in instances in sequence look here 4 more