People who have birds and fish probably set the cage near the tank, but the Duplex stacks the two—cage on bottom, tank on top—in a genius move that everyone except the pets themselves will enjoy.
The fish tank has a deep concave bottom like a wine bottle, so that the bird can fly up "into" the tank. I don't know what kind of enjoyment the fish would get out of this, but I imagine the birds would be entertained, up till the moment they realise that fish can suddenly fly higher than they can. The fish, on the other hand, won't notice a damn thing. Cuz they're fish.
There are some structural issues I have with the design, issues you might have as well. The disc-shaped base of the thing had better weigh a ton to support not only the very tall bird cage, but the heavy water-filled (glass?) bowl atop it. And anyone who's ever owned birds or fish will be wondering about how to clean the damn Duplex—one hopes at least that the fish tank's green gunk build-up reaches the point of intolerability at the exact same the bird droppings do, but chances are you'll be disassembling that thing all the time just to keep your little lovelies from croaking. [Yanko Designs]