The obj file does not "know" that you will be using two maps. This is just a trick you pull on it so that you can re-use the same UV space by overlapping material assignments. The same concept as Tiling a surface -- different areas of a mesh are textured from the same area on the UV layout. This is all done to permit the use of multiple, smaller maps to achieve the same pixel resolution as much larger, resource-hogging, single layered maps. And Poser seems to prefer chewing on several small maps rather than one huge one.