Don't think this is supported "out of the box". However, this will work and is fairly painless: 1. Render out to stereo, close project 2. In a new project, import your stereo file 3. Right click track->Render->Render/Freeze->Render tracks to mono stem tracks In your new project if you haven't saved or anything chances are the mono tracks will appear in the default (Reaper Media) folder. But you can select (click) the mono items and hit Ctrl+F2 to find out. If you're going to do this often I'd suggest creating a blank project for this purpose and loading it in whenever you want to go stereo->2xMono. That way, you can choose where the mono files go (the save dir of the blank project), and you can apply any particular postprocessing in the stereo->mono split chain. z. :)