I have an object with 3 materials, each one of them has their own diffuse normal map and a specular map. I need the object to use only 1 material though, I've baked the diffuse to one picture, works well, but I do not know how to bake the 3 existing normal maps into one just like the diffuse, same question applies to the specular map.
Facing the same problem now, really confusing, but I've just found a solution.
Normal maps contain non color data, and when baking normal map images (e.g. by shader emission, when noncolor is enabled) doesn't gives you what you expect.
You should enable color for each normal map images for baking as a diffuse or emission (of course it changes preview result).
Note that it will not yield exact results, but very close to original, maybe because I'm not using EXR, but I'm fine with the result.
English is not my first language so I hope its understandable.