Their melting points should differ significantly, suggesting an alternative to TLC.
In TLC, could the solid be taylor-made? If the solid has fewer active centers whose spacing matches the ortho- compound, it will interact with the ortho- more strongly than with the para-. Say, if the solid is a polymer, bearing as side chains these same ortho- groups on phenyls, possibly as alcohols to be more sticky. Then, of course, said polymer should better not dissolve in the liquid...