Yonko literally means "4 Emperors" but it's a title so it can be used to refer to each of them individually. In the same context, even though Ryoyoku literally means 2 wings I guess it could be used as a title that refers to both Sanji and Zoro. For instance. "Kaizoku-O no Ryoyoku" - Vinsmoke Sanji or "Kaizoku-O no Ryoyoku" - Ronoroa Zoro ^^