They are equivalent but separate. You join the plan according to where you live, not where you work. Say, if you move from Ontario to Quebec, you will join the QPP automatically. That means all your previous contributions in Ontario will also move to QPP. If you then move back to Ontario (or anywhere outside Quebec), your plan will automatically switch back to CPP. There is really nothing you need to worry about unless, of course, Quebec separates while you are living in Quebec!
