Parsing values in multipass cg shader

Hey, I'm searching for a way to parse values from one pass into the other in a cg shader. In the first pass I want to do "Cull Front" and render the distance of the camera to the backfaces (or save it to a texcoord). In the second pass the same is done with the frontfaces and the difference is rendered to the alpha channel. But if I render it to the backfaces color it gets clamped to be [0,1.0f] of value and if I write it into texcoord2 I don't get the value out of the texcoord in the second pass (though I have to reinitialize the input). Thanks & greetings, Atarax