Mick, the idea is basically to add a duplicate planet with a negative radius. Use a copy of the original planet and terrain shaders but invert the displacement values and adjust offsets to make the top and bottom terrains mirror each other. After that, I use different redirects and displacements for each surface to make them not identical any more.
I like to clamp the 'floor' of the original planet but leave the roof alone so there is a nice, flat ground surface. Obviously leave the second planet atmosphere input empty. That's about it, in short.
I think I could have used more extreme redirects and secondary displacements here as the floor and roof still kind of look too similar, there are differences but not enough.
