@MARK & all: Sorry to interrupt Your questioning ArA, yet, please, please stop with that open-range zoo concept as "the" solution to everything going wrong in the zoo world: even there, You have plenty of clashes between different species. And even if the animals have a lot of "space" to avoid each other, You still end up with "bottleneck" situations at feeding lots or at the stable areas.
@Ara: Sorry, I have to dare to differ to that: if carefully planned ahead and with the animals calmly, competently & slowly introduced to each other, mixed species exhibits do not have to ultimately result in stress. Although several disadvantages, like possible, often not easily detectable permanent stress (the LA zoo made some interesting observations in regard to this-see MAMMALS IN CAPTIVITY), increased risk of interspecific illness/parasite transmission/ malnutrition (animals eating the food designed for the other species and iapt for them) or injuries due to "misunderstanding" (defensive behaviour of one species regarded as playful by other-see famous example Cat & Dog -tail waving) and often more elaborate exhibit design/ general care (f.e. seperated feeding) aka gentle lemur's "devil in the detail" should not be ignored, the interspecific display of animals can have several advantages. Mixed species exhibits can even imitate relationships of species in the wild, like the former Hulman langur & Barasingha Deer exhibit in Hannover, and can make zoo-life
more interesting & healthy not just for the visitors, but also for the animals (see lion-tailed macaques, otters & Orang-Utans in Münster Zoo).
The main problem You have with mixed-species exhibits is the unpredictability, which is often based on the individual characters of the animals kept together; a combination that works in one zoo (I think Melbourne has pygmy hippos & mandrills together), does not work in another (see Yassa's example of Halle), and sometimes combinations that seem to be working out fine can go horribly wrong from one second to another due to apparently no reasons (see the example of Basel-or the many, many more or less fatal accidents in the popular "African savannah" big ungulates exhibits worldwide.). Sometimes the reason why combis don't work can be laughable; I remember the tale of a small gazelle male in an European zoo that developed the habit to consider all females in its exhibits as its own-even giraffes and elands-and started to molest everyone...

There are two ways a "good" mixed species exhibit can be like:
1. The "great neighbourhood": positive interaction (f.e. Geladas & Barbary Sheep in Stuttgart Zoo)
2. The "anonymous apartment house corridor": the animals share the same exhibit, live happily along and don't actually interact much if at all with the other species (f.e. Hutias, Costa Rican squirrels & Azara's Agouti in Munich Zoo)
(BTW:Some zoo professionals even encourage the idea of putting (yet not permanently & unnecessarily!) stress on animals, especially on the ones planned to be reintroduced to make them "fit" for the wild (which happens to be quite a stressful place for a lot of species, especially the ones on the lower end of the food-chain...).)