Observer notify changed to Subject

I am studying about Observer pattern. The model of this pattern can be represented like below picture(source: http://ift.tt/1BZX1h2).

I know when have changes, subject will notify to all observers which register with this subject.

The question here is: When one of the observer has changes,how can it notify to subject and other observer? And follow Observer pattern, is it legal ?

PS: I am using C++ language for implementing