There are two approaches for CR props setup.
1. Single motor with gearbox: Silvestri used in F3A World Championship, 2007, you may find pix in RCU. As I know it is not commercialize yet
2. Two back-to-back motors with a hollow shaft in front motor. ESCs are not issue. Trick is the rear prop has less pitch than the front due to wash airflow after first prop. Call Jarvis of MAXX Prod. He has been played this for years.

Hope this will help.

Luke