1. Create a Stereo Audio track (TDM) or Aux track (LE) in Pro Tools
2. Instantiate the multi-mono Reason Rewire plug-in on the insert of that track.
3. In the plug-in window in the upper right corner, click on the 'link' button to disable linking (it looks like 2 overlapping circles - when active they're orange on a white background, black on grey when disabled.).
4. Click on the box to the left of the link button - this will now allow you to select the Left or Right side of the stereo track and assign a Rewire input to it using the drop down box next to 'Reason' in the main portion of the Rewire plug-in window.