I am currently a "Cadet" with the Federation and a "Serf" with the Empire.
Those are the second ranks ("Recruit" and "Outsider" being the first ranks, respectively).
Interestingly, I have done 2 missions for the Feds and 4 for the Imperials, but each only has given me 2 ranks. The Empire appears to demand more of its navy. I can see many great roleplay reasons to allow players to rank up in both. It certainly is perfect for me playing as a "Covert Operative" for the Alliance. Pretty sure this is as intended, with Reputation being the key to who is hostile toward you. Definitely a good system, it adds a few more layers that way!
Now I just wish I could find a list of all the possible ranks (or join up with the Alliance officially!)