The referendum legislation could have been drafted to contain a clause specifying in which situations it would have been "deemed to have passed". That clause could have contained the requirement for a double majority. Of course, being a non-binding referendum, such a clause could not have had any direct legal effect, but it would have been politically near-impossible to disregard. There is no logical contradiction between the idea of a referendum being legally non-binding and the idea of a referendum with a higher threshold for passing than "50%+1 nationally".
