Question

How can we say that action and reaction forces are acting on two different objects

Answer 1

Newton's Third Law of Motion explains that forcesalways come in action-reaction pairs. The Third Law states that for every action force, there is an equal and opposite reaction force. ... The two forces create an interaction pair on different objects and are equal in strength and opposite in direction

Answer 2

if action and reaction both applied in same object then it will neutralize and hence the force acting wiil be zero because action and reaction are equal and opposite in nature, so they will cancel each other.
ex- if u sit on a chair with folding legs and pick the chair with your hand by holding the handle of the chair it will not work.
because the action and reaction are actiong upon the same chair.