eh... I could see it. From what I'm gathering it's a case of a half bath with two doors in between the two dorm rooms. So if you go into the shared bathroom, the first thing you would do with be lock the other side door, so the other person didn't walk in on you. And if it's locked you would just wait. So at no time would the two of you likely be in there at the same time.
That wouldn't explain why they didn't see each other all the time entering or leaving their dorm rooms. Or if Dorothy really lives in the room next to Amber, that she never noticed Amazi-girl going out the window next door...