Aquarium/Terrarium Combo Question.
First of all, what you are talking about is called a paludarium if you wanted to do more research for yourself. Second, in a 20 gallon tank, the water area would have to be much bigger than that if you wanted to keep fish in it. Third, poison dart frogs or a type of newt would be your best option for terrestrial animals.
For the land base, I recommend using the website Josh's Frogs. Buy their false bottom substrate, it's amazing. However, you could use plain aquarium gravel instead (But nothing with limestone). Then on top of the false bottom, place Josh's Frogs ABG mix substrate, as plants grow the best in it.
A divider is not necessary, as long as the water only fills up the false bottom media and doesn't flood the tank. ADA Aquasoil is a good substrate for the water portion.
So excited to see this build, paludariums can be quite beautiful!