The programme ATLC calculates the impedance of transmission lines of totally arbitrary cross section. The two conductors can be coaxial, run side by side, or just about anything you can think of. The programme will allow for more than 16 million different dielectrics, with any permittivity you want.
As of version 2.01, support for multiple dilelectrics is not included, but this will be added. At the minute only air is supported.