Like @Laci_K already said: There is no split-surface due to the fact that there is no “Inside” of the mesh, because it is not closed. No “Inside” means that there is no material that can be computed. In fact the mesh is no body (with a volume) - it is just a shell with zero thickness. If you are still working on the occulus-controller you may try using the file I provided where I gave the surfaces a thickness of 0,01mm - there will be an tiny split-surface visible.