Hey Karl, good luck with that! I can't offer much help since I have never tried something like that, but I was very inspired by this article:http://badmanstropicalfish.com/vivarium/vivarium.html
and one day I will have time, energy, money, and room to set up something like that! Maybe in 35 years
Anyway... I think you can use aquatic substrate for the terrestrial part. I assume you want to plant the land part... then it would be something like hydroponics, where you have the plants growing in gravel, fullers earth, whatever, and you either wet it every so often, or the water is collecting on the bottom for the roots to access.
Let us know how things turn out!