Ideally, you should install motion sensors at every entrypoint of your house including upstairs windows. Install a motion sensor near the shared hallway or stairs along with a window sensor on any window you might have for better security.  It’s common to think or feel burglars won’t choose upstairs as the intrusion point. But that’s a mistake. You should secure all windows and entry points to other floors just as you’d secure your ground floor for maximum security.