It would make sense for some moves I guess, but as the others stated, to many problem in reguarding to type weaknesses, resistances, and a possible modification on the entire playstyle of battle. But something similar I would like to be implemnted would be combining moves. For example, a lucario has Close Combat and quick attack. Quick attack would provide as a catalyst move and give close combat +1 priority. Anyway maybe if there were new moves added for the specific purpose of a dual type attack I think that should be left alone. But the only pokemon I can see using that type of move would be arceus's judgement or genesect's techno buster.