Aht Urghan Whitegate has multiple sets of arena doors for a pvP system that was never implimented.
It should be made clear that it's not actually known what the other Colosseum area was supposed to be for. They never really said anything about it. Many people assumed it would be a duel area because of the extremely tiny area (same size as 1 of the four blocks of The Pit) but that was only speculation.