Help in dynamically creating a transparent area on a movieclip

I have a .jpg graphic in a movieclip on layer two (1024/768). This is used as a background.

I have a .jpg scrolling movieclip on layer one. It is a very wide graphic used for displaying a panorama.

I would like to be able to create, dynamically using actionscript, a transparent area on the layer two movieclip (create a window) to view the underlying movieclip.

I believe this is possible using the Rectangle class in the flash.geom.rectangle but I can’t figure out how to attach it to the layer two movieclip or how to make it transparent so shows the underlying scrolling graphic.

If someone can help, I would sure appreciate it. I am a beginner in Flash and trying to learn.