I think you could do this two ways. Since the shadow is color as the background. You could use the Perspective/shadow effect and set the shadow opacity to 100% and use the background color to be your shadow.
Another way to do it would be to duplicate your front circle layer, remove the position keyframes, offset it to the shadow position and parent it to the front circle. Now change your large circles Track Matte to be Alpha Inverted and it the matte circle will cut a shadow out of the large circle.
I’ve attached a project with both methods.