At the time of the founding, as you notice, citizenship of a couple tracked citizenship of the father, so one could figure out NBC by either looking at the couple, or at the father.
These days, with the family structure disintegrating, who knows. I think it would take a few hypothetical examples to figure out a "just" rule. Intuitively, I'd look at the parent who raised the child.
Well in this case they both raised Ted, so that does not help clarify. Like I said, this case is not settled. Will the courts take it up? No one knows, however, it would not surprise me if they did.