The Tosefta at the end of Masekhet Keritot asks:  Why does Aaron precede Moshe in this verse, whereas Moshe usually precedes Aaron?  The Tosefta answers that Moshe and Aaron are equal.

This statement is puzzling. 

1. Isn’t Moshe clearly superior to Aaron in a number of areas?  The Torah explicitly says that Moshe was the greatest prophet that ever lived.  It also says that he was more humble than any other man.  On what grounds can the Tosefta claim Aaron's equality?
2. A Tosefta in Megilla (3:13) refers to Moshe as greater and to Aaron as secondary:  “Just as the lesser person acts as the interpreter for the greater one – as it says, ‘Aaron your brother will act as your interpreter’ – so also the reader of the Aramaic translation (this was common practice in the times of the Talmud and is still done in Yemenite congregations) should not be of greater stature than the one who reads the Torah scroll itself.”
3. The Mishnah in Keritot (6:9) lists a number of pairs (father and mother, sheep and goats, pigeons and doves) where both items are equal.  Why are Aaron and Moshe not listed? 

The last question is less severe. The Mishnah might only list comparisons with halakhic ramifications, and the Moshe-Aaron comparison has none.  However, questions 1 and 2 still stand; in a number of sources Moshe is clearly superior.

Apparently our Tosefta in Keritot is making a much more limited statement.  In general, Moshe is superior to Aaron.  However, the verse here refers to Aaron and Moshe’s role in the Exodus, where each one played an equally important role in the mission.  Moshe was much more centrally involved in actually taking the Jews out of Egypt, as will be evident from subsequent parshiot, and Aaron's role was secondary.  Aaron, though, was the main spokesman in interactions with Pharoah, where Moshe was secondary.

Yet the Torah, one verse after another, switches the order of their names.  When it speaks about the actual Exodus – “to whom G-d told, ‘Take the Jewish people, all of their hosts, out of Egypt” – where Moshe was central, it lists Aaron first – “Aaron and Moshe.” (Shemot 6:26) Then, in the next verse when it talks of speaking to Pharaoh – “They are the ones who speak to Pharaoh the king of Egypt . . .” – it lists Moshe first – “this is Moshe and Aaron.” (Shemot 6:27) This switching of the names actually teaches a lesson. By listing Aaron first concerning the area where Moshe was central and listing Moshe first in the area where Aaron was central, it makes it clear that both had an equal role in the mission.