You can do either of the following:
1. Set the output modules to render to movie file formats, such as QuickTime. Render engines can not collaborate on rendering to movie file formats, because only one application at a time can hold a file open for writing.
2. Set the different render engines to watch different folders. In this case, the render engines can't render the same project (unless you duplicate it into each watched folder).
OK thanks. So if I understand correctly, one master AE with multiple comps being rendered in one queue, can not be distributed on a comp by comp basis to several engines all looking at the same watch folder?
Correct. The After Effects render engine isn't quite that smart. Each instance of After Effects is independent, and when working on the same project in the same watched folder they communicate with each other via the render control file (projectName_RCF.txt) about which instance owns the rendering for which comp or which frame of a shared comp.
OK thats clear, thanks Tim!