@Chlidonias, I wouldn't call a sun bear a "big" carnivore, they are quite small and they don't eat 22 kg binturongs... They also use different places in the exhibit. The binturongs are mainly high in the dead trees, the sun bears spend much more time on the grass/lower trees.
@lintworm: Sun bears may be the smallest bears but they're known for their nasty tempers (attacking without provocation) and vicious claws/canines. While the bears don't actively prey on binturongs, should any conflict arise, the binturongs don't stand much of a chance.
@Zooish: Agree, but in Arnhem the mix - as lintworm wrote - works. The only (very) disapointing thing about the exhibit is the indoor part (as you can see in the picture above). It is really, really small (even when Binturongs and Bears should each have their own indoor part, which I couldn't find out yet, because for visitors only this one is visible).
There was a high-up, binturong-sized pophole visible through the back fence of the exhibit. This seems to suggest the binturong-only housing is behind this on-show area, but that's only my inference.