I feel a difficulty in understanding SP at fundamental level which is somewhat related with twin paradox. Let me take a thought scenario: in empty space or vacuum two boxes(A and B) with their own light sources attached are separated from each other at a far distance r initially. If they are approaching to each other in different inertial reference of frame (they know this by receiving light signal sent from other), they would say ( assuming they can think and talk) that I am at rest and you are moving so what A think is ##v_A=0, v_B=-v## and what B think is ##v_A=-v, v_B=0##. My understanding is that under SR two views are equally valid because SR only talks about the relative motion by comparing states between two inertial reference of frames. In such cases, if my logic is right, then it generates a fundamental problem. Just single global event( two approaching to each other) can yield two different views which are valid under SR view. The first postulate of SR says that "1. The Principle of Relativity The laws of physics are the same in all inertial frames of reference" and it conflicts with the result of two boxes approaching. Can a single event create two different result? Where am I wrong?