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).
If you want the planes to be in 3D space, then you could try using a material like this on the plane with the green screen:



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.