Rendering two textures with blending and alpha test

Hi guys,

what I am looking for is the following... I have a circle on a square image, alpha is 0 at the corners

http://dl.dropbox.com/u/1401029/circle.png

and also a *square* shadow, alpha is 0 everywhere else

http://dl.dropbox.com/u/1401029/image.png

Well, I would like to have as final result a blending of these two renders, plus tha shadow not being rendered outside the circle

How could I achieve that? :)