Preventing "double" or "stacked" shadows

[davision]

First of all -- this is not a GI animation, so hopefully this simplifies things a bit.

I am rendering a full color background. The sun's angle is low so I have many long shadows across the ground plane.

Next I render my moving "actors" which are also casting a shadow on the ground plane (which is a matte object on this pass).

When I composite the BG and FG, I get "double" dark shadows where the long shadows from the enviroment (trees and such) intersect with the actors' shadows on the ground plane.

I know why this is happening, but I'm not sure if I know the best way to prevent it. Can anyone provide some tips?

[RErender]

You can render a seperate shadow pass for both layers and subtract the base shadows from the actors shadows.