Or HaChaim on Leviticus 11:1/אור החיים על ויקרא י״א:א׳

1 אל משה ואל אהרון, to Moses and to Aaron, etc. Perhaps the extra word ואל in ואל אהרון is intended to put Aaron on the same footing as Moses in their duty to communicate the laws of forbidden foods to the Israelites. G'd also addressed them as equals when He said: דברו (pl) אל בני ישראל. We find something similar in Exodus 12,1 where G'd used the same wording to introduce the legislation of the Passover. This interpretation is possible seeing that from an exegetical point of view we have other verses which teach us that Moses was to teach Aaron the law before the latter was able to communicate it to the Israelites.

2לאמור אליהם, to say to them. Torat Kohanim on our verse explains the word אליהם, "to them," as a reference to Eleazar and Ittamar. The idea is that these two sons of Aaron were to be acquainted with this legislation before it was to be taught to the entire people. This is based on Eyruvin 54. Perhaps the Torah had in mind to emphasize a point made by the prophet in Maleachi 2,6 that the lips of the priests are to preserve knowledge and that people seek to obtain knowledge of Torah from the priests. The words לאמור אליהם may also reflect what I have explained in connection with Exodus 20,1 that the reason for the word לאמור is that the word emanates from the Supreme G'd and that G'd made an angel out of it and that it was that angel who spoke to Moses. [The concept is that the "word" is something in between a totally abstract concept and something which has at least a semblance of something physical about it. The absolute abstract, i.e. the Essence of G'd, cannot communicate with a human being who is part body by means of something entirely abstract. G'd therefore created an intermediary called an "angel" to accomplish communication by word of "mouth." Ed.] The word לאמור refers to that angel who would communicate with Moses and Aaron. This explanation is not at variance with what our sages said in Torat Kohanim at the beginning of the Book of Leviticus that G'd communicated verbally from the inside of the Tabernacle only with Moses, not with Aaron. What it means is simply that the angel who addressed Moses used the plural form דברו! when addressing him instead of saying to him דבר! (singular). Moses relayed the words to Aaron before he spoke to anyone else.