I always thought that creating a mask layer inside the mask layer was impossible in flash. Recently I was working on one of my projects and by accident I've created mask within the mask layer.
It's is very simple to achive. 

1. Create a mask layer
2. Create a simple layer put something on it and then drag the key frame on the mask layer
3. Drag it under the mask

 

3 Easy steps. You can check out my "how to" video on You Tube. Here is the link: http://www.youtube.com/watch?v=Z7bAzxPuQME