Of course, Moses was not really God, but in the teamwork aspect of their working relationship, God is clearly pointing out that Moses was the leader, even though Aaron would be doing the bulk of the speaking—at least until Moses' confidence, his faith, increased to the point that he no longer worried about being slow of speech. Moses would be in the position of issuing the orders. Aaron would be in the position of submitting to what Moses said.
Moses was in the position of God to Aaron, even as God was to Moses. Moses was God's prophet, but Aaron was Moses' prophet. A prophet is one who speaks for another, who speaks the words that the other put into his mouth—a simple arrangement, easy to understand.
John W. Ritenbaugh
Unity (Part 2): God's Pattern of Leadership