It is the normals on the hair prop. You have morphed layer that was just intended to cover the back of the head into visibility. To be visible from both sides at least one layer facing each direction is needed. Doing this will change the number of verts and thus make it a different obj. The two face utility does not just add a new set of normals, it also duplicates the polys and doubles the file size. Open the hair prop by itself. Use the grouping tool to isolate the part you wish to see from the front. export this as a prop. Import it back,and reverse the normals - make sure it is in place and save the combination as a new hair prop.