However, seeing as they are both open from 09:00 -18:30, I'm sure I'd be happy with a day each; personally, I don't spend much time exploring indoor exhibits unless the weather is poor.
Although indoor exhibits *are* a large part of why Zoo Berlin takes more than a day to see properly, this is nowhere near as much of a factor for Tierpark Berlin where it is the sheer size of the collection which has the greatest impact. We are talking an exhibit density similar to Chester Zoo in a collection two-thirds the size of Whipsnade Zoo. I strongly suggest that unless you want to a) view the collection at a jog and b) omit any revisits to no-show species even 9 hours or so will not be sufficient for the latter collection