Use Blend & Comp to apply basic blending and compositing between two inputs, and their mattes, over a background.
To access the Blend & Comp menu, use:
Timeline, then use ConnectFX (See Creating ConnectFX).
Tools, then select from the menu.
The Modular Keyer, then select a node from the Node bin.
This node accepts up to two front and matte clips, as well as a back clip, and outputs a result and outmatte.