The males (Kiburi, Isiro and Shomari) are kept in one paddock, whilst the females you can see in this picture and last year's calf Hazina are kept in the other. Sometimes they are kept seperate if required, but usually they get on well in a social dynamic.
According to the most recent Marwell Animal Inventory there were two males and two females in the collection as at 31st December 2015.
Neither did I, Loxodonta Cobra, the only zoos I've ever encountered Okapi in collections (Bristol and Chester) have the mature adults strictly separated to encourage a solitary life style, except for breeding purposes and maternity care!