I could get the result that I wanted (double-layer glass object). Having the mesh separated into two parts (inside and outside) did not solve the problem. I tried using the mesh visibility to control the alpha, while the material of both meshes is the single glass material. It helps to control the inner and outer of the glas, but the clue was to make sure to activate the “seperate culling pass” on the glass material, because only then the normals don’t get randomized.
I did not find any hint on the normals behavior (randomize?) in the documentation, though.
Thanks for your help, I keep sailing on line-of-sight.