Some zoos have successfully kept wolves and bears together ;-)
I would think that at the very least our large African herbivores can be kept together. Maybe with the exception of elephants, though all-female herds do well with smaller animals in their exhibits on occasion too. Mixed-species Savannah paddocks are very common in zoos (where I work we have giraffe, crowned crane, ostrich, zebra, greater kudu)
My presumption is if that the enclosure is suitable for two animals to live in side-by-side and not be unhappy with any of the enclosure's elements (such as terrain type, space, what have you) and they do not naturally get into altercations (e.g. predator and prey, two large predators, aggressive herbivores...) they will be able to share an enclosure quite happily. You could create some interesting combinations this way; lemurs and peafowl could be a possible combination, for example.
On the other hand we may get a system where enclosure partners are considered 'unsuitable' for whatever reason and their very presence makes the animals unhappy, who knows. ATM I predict the above, potentially with an overall 'population' need threshold like in JWE.