Then, Arya looks in the mirror, only to see.... Tyler Durden - DUN DUN DUNNNNNNN.
Good theory though, although I don't like the over complexity of the house of black and white, Arya having multiple personalities etc. On the preview to ep. 8 we see Arya presumably running away from the waif, so there is more to the chase. I say either of these scenarios are going to happen for sure.