This Is The Word Of Yahweh (Leviticus 15:1).
“And Yahweh spoke to Moses and to Aaron, saying,”
The variation between Yahweh speaking just to Moses and sometimes to both Moses and Aaron, is a sign of the authenticity of the narrative. It is unlikely that an inventor would have introduced such variation so spasmodically. Again it is emphasised that we have here Yahweh's words, but here to both Moses and Aaron. Since Aaron's advancement to High Priest Moses wanted him more involved, especially with matters related to the tabernacle.