How can I get the backface depth?

I’m writing a SSR shader.
I need to get the depth of the backside to further get the thickness of the object.
How can I get it in Urho3D?

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Render the scene twice? Render with backface & front face culled in 2 separate passes into 2 different depth buffers.
With DX11+ UAV or OpenGL’s SSBO you could write to two depth buffers in one single pass based on orientation of the fragment, but I think Urho doesn’t support either of those two.