hm... i don't see any problem. i would have to check it myself with the blend file.

The blend file is huge. I will make a smaller one. But, for now I have to go.

Thank you very much for your time and efforts!

Guess what? I got it working!
I have to save the blend file with the second skin, before I export it.

Nonetheless, a further question:
- Does it need a special material setting to see the combination of the skins ingame? At least, the preview in MED doesn't show both at once.

med doesn't support showing both at once. i think the engine treats the second skin as light map by default. if not then you have to assign some light map material.

It works even within MED's preview - my skins had been not different enough to recognize it! Sorry!

How can I see the mix of the two textures already in blender as I see it in the acknex engine?

add a texture using the second skin/map set the mix type to screen/overlay or something I cannot remember. Make sure the texture input is using the correct UV...UVTex or UVTex.001, but You know what you are doing so you prabably re-named it shade or something.

I don't know at all what I am doing, but I want to know!

I find this texture, material, image, blend etc. settings extremely confusing.

I'm getting closer to a solution, although I don't understand all things.
The "lighting_baking.avi" from Yo Frankie DVD helped me. The tutorial is extremely fast, I had to stop it several times, and I compared the settings seen and changed in the video to mine in blender and vice versa.
I see the mix of the 2 textures in the material preview now.

If anybody can give an advice how to "apply" this mix to the object in the 3D view as well, it would be appreciated!