I found this as the 'best solution' on another site:
Use the Utility > Gain plug-in to separate the stereo track into its components. You can then either bounce the resulting audio to get two files or you can route the signal to two different busses, each with the Gain plug-in attached, one dealing with the left channel and one with the right.
To do this, though, make sure that the Aux channel associated with each bus is set to Mono. You can do this in the Mixer pane by clicking on Mixer at the bottom of the Arrange WIndow.