Im pretty sure Franchi makes or made an O/U "paradox" (at least a sorta paradox. As I recall one barrel (I think it was the top), was rifled or partially rifled. If I remember correctly the bottom barrel was smooth but could also shoot standard slugs.

Back then it was out of my price range, and I didnt have a great urge for such a thing. Today is different but of course I cant find one