It is possible to to route Submixes to a Return and both out the Master Output. This can all be done in the Channel Mixer. After creating your Submixes in the channel rack of your Program, open the Channel Mixer. Set the Mixer type to Return and tap one of the open insert fields to load an effect.
Change the Mixer type to Submixes, and tap on the Send tab. You'll see all of your Submixes displayed individually with the ability to adjust the send amount.
Hope this helps! Let me know if you have any questions.