Creating a ClearMask-Texture

Hi, i am very new to shader stuff and did my search but i couldn't find a solution to my problem anywhere else and i think this is doable with shaders:

i need to copy the 'clearing', that a camera observes, to another camera.

my idea was that if i could generate a monochrom "clear-texture" from the first camera and submit it to my 2nd camera it should do.
-similar to this i guess:http://docs.unity3d.com/Documentation/ScriptReference/DepthTextureMode.DepthNormals.html?from=Camera

-the second camera can not be in the same place as the first and i need the same pixels cleared, therefore just using the same masking procedure won't work

Edit:
I am Using this DepthMaskShader and SetRenderQueue for the masking
http://wiki.unity3d.com/index.php?title=DepthMask