Two practitioners, one treatment room
A room shared by a physiotherapist in the morning and an osteopath in the afternoon works until the morning runs late and the afternoon starts early. Somebody finds out in the corridor.
Give each room a sub-calendar that refuses overlapping events. A booking that would put two people in the room at once is refused when it is saved, and repeating clinics are checked for clashes across the coming year, not just their first week.
The morning overrun would have put two practitioners in Room 2. It is refused on save.