i made a Barrel Model, uv unwrapped it, assigned two material and then copied that whole object.
Delted all objects from the scene except that two.
I deleted then the two textures and used instead a new one as you can see from the screenshot
Then I selected first the textured and second the untextured object. Then I clicked the Bake button.
I tried it before with a simple cube, what wokred for me excactlz that way.
However the result with this model is a not quit right.