I see no reason why they couldn't exist side by side but at the same point, I don't see a need for one if you have the other. Unless the auction house has some unique properties like being global and having unique penalties associated with it, it would be almost identical to the market ward system we have now.
At launch I could see the need for it, the wards sucked. Now the wards are functioning as I think they were intended to so there's not a lot of need at this point.