There are several approaches to differentiate the two zoos and exclusive species do not even have highest priority.
-city zoo vs. landscape zoo or zoological garden vs animal park: Berlin Zoo tries to show all the highlights in a comparatively(!) condensed area. There is a high focus on traditional as well as modern architecture and gardening (prominence of flower beds or lavishly paved paths). Berlin Tierpark in contrast aims at presenting animals embedded in naturalistic landscapes. They even try to hide big buildings behind plants (e.g. the listed Brehm building!), artificial rocks (elephant and monkey building) or graffiti (main entrance, giraffe building). The main ways get embedded in more naturalistic looking vegetation and complemented by smaller 'adventure paths' with different colours and substrates. A clear exception is the immediate castle park area with its fountains, water channels, visual axes and flower beds, of course. So the zoo seems more spectacular with short ways and a high frequency of highlights. The Tierpark rather represents a great place for a family trip or extended walks in nature as if to simulate an actual journey, safari etc (park trains etc. are to limit the walking for some types of visitors, though).
-systematic approach vs. geographical approach: Berlin Zoo generally groups related animals so you get to discover patterns and appreciate variety within seemingly similar animals. Tierpark Berlin groups animals according to their habitats so you walk through Serengeti, South East Asian Islands, Himalaya as well as regions in Europe, North and South America or Australia. So in Zoo Berlin you will rather hear children say the classic "Lets go see the elephants, monkeys, lions..." In Tierpark they may originally say so as well, but regular visitors may increasingly start to think in continents or even habitats.
-complementary/ exclusive animals and attractions: Zoo Berlin has elephants and rhinos from Asia while Tierpark displays their counterparts from Africa. Both show different species or subspecies of giraffes, bears, tigers, leopards, monkeys, deer, rodents, birds, crocodiles etc. In a similar way, Zoo Berlin has a show with sea lions, Tierpark Berlin one with birds of prey.
By the way: These are only tendencies, whereas none of this is taken to the extreme. The administration mainly gives pragmatic reasons for this coordinated development (limited space in Berlin Zoo, try to have people visit both), but they also claim to make both as attractive as possible by themselves. And you can positively be sure that Zoo Berlin won't sacrifice anything just to help Tierpark out, as they strongly rely on generous donations by local residents who are not having any of that (the other way around it is far more likely as Tierpark Berlin is funded by public money). This is also where the 'no duplicate policy' ends: Both Tierpark and Zoo display polar bears, Berlin Zoo is just about to get a breeding couple of Red Pandas from Tierpark, so they will have 1,1 each. And Zoo Berlin will not give up on any of their 4 ape species, but Tierpark Berlin can show the missing Orang subspecies in the future. And they will get sea bears and other attractions as well, none of which will actually be completely exclusive (there are rumours about koalas, but I don't quite believe Berlin actually will end up having them in Tierpark Berlin or only there).
If this last bit sounded critical, it is because I don't really like some of these internal and completely inofficial 'zoo first' policies. But I still think that both zoos are and increasingly become very attractive. And both represent quite complete collections, while Tierpark Berlin still has some obvious gaps but also some amazing rarities as well as some generous and simply beautiful enclosures. And its definitely worthwhile to visit both of them (our family actually has a combined anual family ticket for Zoo + Zoo Aquarium and Tierpark which costs us less than 200 Euro for 5 people!).
and the polar bear cub is at Tierpark.
