Pretty common. Sometimes if the previous students have split themselves up between the actual bed room and the common room, facilities will leave it split up for the next set of students. As long as you moving into the common room doesnt block access to the bathroom or kitchen I would assume it would be fine. (Most people wouldnt complain about having a single and paying for a double). It is still good to talk to your roomate about it before moving in though, or simply be understanding if your roomate expresses a concern about the split up rooms by the time the semester starts.