In this demonstration Colin shows how to add soft pastel on top of pastel pencils in a background. The pastel pencils chalky texture acts as a good base for the soft pastels. It would be very difficult to lay a soft pastel base and then add pastel pencil over the top.
The set used in this video is made by Faber-Castell and can be found here. Watch Colin’s demonstration below to see how to improve your backgrounds with soft pastel: