Create a Shape Tween on the Circle Layer.
That completes the Motion Tween for your first mask. You're welcome to add multiple frames for more complex animation but for this example it will be fine.
Create a New Layer above Circle 1 Layer. Name this Layer Background 2.
Copy the Background image on the Background Layer and Paste it on your new Background Layer 2 at Frame 1.
Create a New Layer above Background Layer 2 and name this layer Circle 2.
Right click on Circle 2 Layer and select Mask.
Go ahead and create a new shape Tween on this layer using the circle shape you used earlier. (You could use any shape if you like, I'm just working with circles here.)
Step 9. Repeat steps 5-8 to add more masks to your movie. The thing to remember is to reuse the main image each time you want to add another mask.