Blizzard is willing to make changes if it seems enough people want it, so there is always a chance that they would unify the AHs on each server. But there's a pretty clear lore/roleplay/rules reason for having them separate, seeing as you are talking about two sides that are at war. The goblin AHs show how the goblins profiteer from the war and the separation. I'm not sure that there's enough support for the idea, especially since Blizzard would probably rather deal with faction imbalance by promoting ways to bring the population levels closer to each other.