after painful hours of trying i finaly managed to get files with multible meshes and multible uvs from cinema4d to unity. thanks to modo.
- in cinema export two files as fbx each one containing one uv set.
- open both in modo, create a second uvmap in one of the files.
- copy the uvs from the other file to the new uvmap. (texture > uv operators > copy)
- export again as fbx and import into unity.
voilà!