I *May* have got this wrong.. (and please correct me if I have!) but I'm sure I only remember 1.1, the previous daughter from that pair and this new baby from my November visit so possible female two has died or left the collection??
They are actually two different species. The mandrill, though vulnerable in the wild, is more common than the endangered drill. The mandrill is also more common in zoos and if I can remember correctly, the drill population in captivity aren't doing so well.