This is correct.
You should have a total of three codes, printed on two separate flyers. There was a double-sided flyer with the standard key bindings on and the Standard Edition code on the back, and then a second flyer which was a single sheet of paper with two codes: one for the Imperial Edition and the other for the Explorer's Pack.
You enter the first code to confirm you've bought Elder Scrolls Online, and then enter the second code to upgrade it to Imperial Edition.