Actually it has been proven that Dream Sharing doesn't exist. Because in Multimedia, when they assigned 2 designers the same project, they did their own form of the project. While when you looked at the 2 pieces, they exactly matched the description, the 2 forms of design visually couldn't have been more different from each other. Never in the history of design have 2 different art forms done by designers matched perfectly, there is always something different about them, if you look at them from a designer's eye. That is because everyone's form of visual design is just as unique as a person's handwriting.
You seem to have a completely different concept of dream sharing then most people do. Those who dream share will share the dream but will share it from their different perspective views, when someone says the they shared the same dream, it doesnt mean they did so from the same body in that dream. They each are seeing out of different dream eyes or dream bodies. So I dont see how your example disproves the possibility.
So say like in the dream I dream shared with my daughter. I didnt experience getting my head bashed by 3 people, she experienced that and I saw this happening to her from my perspective. We both experienced the same setting too for this scene.. outdoors, grass etc etc In her point of view, she yelled for help. In my point of experience, I heard her yell for help.
Anyway, dream sharing doesnt usually mean experiencing being the same person in the dream and seeing the same things from the same angles etc etc (thou I guess that could well maybe be possible).