I don't think the Sumatrans usually have access to both of the large enclosures - one at a time seems to be usual, although the gibbons may get access to both. I think they may need to give the vegetation some rest, one of the snags of the natural look.
Alan
On my only visit since RORA was opened I saw the Sumatrans in both outdoor enclosures. Mind you, they only went outside very briefly. I can see why they may need to rotate their use though if the indoor enclosures are anything to go by...
The Borneans at Chester seem to be rather a 'non-exhibit' at present....