There's a command line approach to this that, once you get it all set up, is a little more straightforward.
First you need to install Homebrew, the best package manager for OS X out there. Open up Terminal and type:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
With Homebrew installed you can install all kinds of cool command line tools using the
brew command. We're going to use it to install sox, "the Swiss Army knife of sound processing programs":
brew install sox
sox installed we can split our audio file in to two mono tracks with one simple command call. First, from GarageBand, export your stereo track to disk as a wav file. With the file on disk, open up a Terminal program and change to the location:
And then run this
sox to command to split the file in to two mono tracks, one for the left channel and another for the right channel:
sox My\ Song.wav My\ Song.L.wav remix 1
sox My\ Song.wav My\ Song.R.wav remix 2
sox will keep the
WAV settings the same as the source material so you should be able to, quite easily, drag and drop these split files back in to GarageBand as mono tracks.