An ERJ-175 First Officer reported lining up with the wrong runway on approach; due in part to deviation from SOP with the Captain (PF) entering data into the FMS.

Narrative

On approach we were trying to guess which runway we would get. I thought the right runway; the Captain thought the center. So we went with the center. The Captain then proceeded to enter the approach into the FMS even though she was acting as the flying pilot. After we got closer; they assigned us the right runway. I started to change the approach in the FMS; and at the same time; so was the Captain. I am unclear after all this time has passed; but it was something like I hard tuned the frequency and when I looked over; she was activating the approach. Finally; a few minutes later; Approach Control switched our runway to the center. We were getting busy at this point. From what I remember; I again went to hard tune the frequency; and at the same time I saw the Captain doing something in her FMS. I assumed that she was selecting the new approach as she had done twice before. But the approach for the center was never activated; so I guess not. After that; we were cleared to join the localizer. The next thing I know; Approach Control is calling us to say we are on the the right runway localizer; not the the center localizer like we should be. So; the Captain corrected visually to join the the center localizer; and there were no further problems. Once on the ground; the Captain called the Controller to clarify what happened; and apologize for the mistake. She told me that the Controller had not experienced any problem or loss of separation because of our error. A clear separation between pilot flying and pilot not flying duties would have prevented this mistake.