# Viewing background through alpha channel of foreground object

I have set up two planes, one in front of another, and I want to see the back plane through the alpha channel of the front plane. In the node editor I use the render results as input to a key node so that the green screen is turned into alpha, but it appears that it's only transparent to the plane itself. Have I reached the limits of Blender?!! Is there a way to do this? (see screen shot).

This mixes between a transparent shader and a emission shader based on the difference between the the second color input of the mix node and the pixels in the image. Pixels which are within a .07 tolerance of the green color in the mix node will be transparent, other pixels will be opaque.