Or HaChaim on Exodus 6:27/אור החיים על שמות ו׳:כ״ז

1 הם המדברים אל פרעה מלך מצרים, they were the ones who talked tough to Pharaoh the king of Egypt. They spoke to him as does a king who issues decrees. The Torah also wanted to remind us that though G'd had instructed Moses to speak, Aaron spoke also, as I shall explain in greater detail on verse 28.

2הוא משה ואהרון, they are the same Moses and Aaron, etc. The Torah wants to convey that Moses and Aaron enjoyed equal status; this is why sometimes Aaron is mentioned first whereas other times Moses is mentioned first. The Torah makes this point doubly clear by referring to both of them in the singular i.e. הוא, as opposed to הם. The Torah uses the plural also i.e. הם המדברים. This is because Moses and Aaron did not speak simultaneously but they alternated when speaking to Pharaoh.