Thanks for the answer.
Unfortunately, this means that every object at the scene should be unwrapped at the same time in one UV layout to be seen poperly. If I try to unwrap one after another separately, Blender automaticly scales the UVs of this one object to the maximum size of the UV space and colours it red/orange, and it does this always after adding a new seam. So I can’t really see whether the seams are marked correctly or not. Or maybe there is some way to lock the size of the UVs after scaling them down?
