What do you mean by dividing it? Like the first thing that came to mind was cutting it in half, but now that I think of it.. you could insert an acrylic wall smack dab in the middle. Granted you don't have to have THE perfect shape to make it fit, but enough where the fish can't get through to the other side

- I would also suggest having this wall possibly not see thru, unless its dark enough in there to not matter.