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!