I think it was just Michael who was the target. the statement of "Just one?" seemed more like a boast to impress Altobella. There was no advantage to anyone to assassinate Mary. Mobsters kill for mostly business reasons and Mary was no threat to that. And there wasn't really 2 assassins to kill Michael. the son was more of an assistant to make sure the weaponry was there. he didn't seem to be involved in the actual killing attempt on michael.