Remember... Oneness isn't necessarily Modalism. For example, in Oneness theology the Logos (mind, plan, will) of God was made a flesh and blood human being, the man Jesus Christ. And from conception... the Father was manifest and revealed in and through this human being, the Son. Therefore, prayers between the Father and the Son are prayers between the man Jesus Christ, and the Father with whom He is one.
Oneness believers would argue that the argument you present illustrates that the divine persons are so separate as to be two individuals who can speak to one another. Thus... two divine persons, beings, or "Gods". To the Oneness believer the prayers between the Father and Son are prayers between the human and the God with whom He shares absolute oneness with.