I received my interview invite along with my secondary. So, your situation is pretty common. Don't freak out, be happy! If possible, send in the secondary at least two weeks in advance of your interview. You want them to be able to review your application beforehand. Also, you want to be complete before the interview, so they can make a decision soon thereafter. The worse part is waiting, you don't want to wait to hear from them, trust me!
Best of luck!