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It seems like such a farfetched idea so if it would happen I can almost guarantee that it will be specific legendaries. I could see Newtwo having it if he's Psychic/Fairy but I doubt that.
Actually the Pokemon gospel says that dual type moves might exist, but I bet they would just be moves like a Fire/Ice Dual Chop where the first hit goes Fire and the second hit Ice. I'd say that'd be pretty cool although they'd probably suck competitively.