I have done both Berlin Zoo and Tierpark many times in 1 day. It's a matter of starting early and staying until closing time.
However, when you can spend 2 days per zoo, I did find that an added value. Especially if you like the smaller species in the big complexes. 2 days of Zoo Berlin with one day focusing on the Bird House and Pheasantry, and the other on the Aquarium-Reptile Building, that was actually perfect.
Also in the Tierpark, I was able to spend a lot of time in the Pheasantry, searching for all those laughing-thrushes. Now, though, the latter is not as extensive as it used to be, the snake farm and crocodile house have even disappeared, and the Elephant House is still under construction. There are large sections with bison, camels, llamas and relatives that don't add much value to me, and which I quickly pass by in a run.
Have a cup of coffee on arrival, enjoy the peace and quiet early in the morning, look at the map and plot a route.
All signs are in German and English, and navigation between the larger sections is pretty easy.
Wear good shoes.