I was looking for that demonstration, and I know how to start: As they aren't contain in each other I have to take a point in each one that it is not in the other one. So they (the points) will be in the union of both of the mainfolds. So I have to proove that the half point of them is not in the union. That's all.
Thank you so much (I hope you have understood my English, I am from Spain)