Constant activity or change is main characteristic of life and physical existence. Without diversity and opposites interaction and activity is impossible. ABSOLUTLY THE SAME can't have interaction and activity (particularly of consciousness).
If we imagine only one man (hard to imagine) or SUBJECTIVE CONSCIOUSNESS without anything outside itself or OBJECTIVE CONSCIOUSNESS; what that man (SUBJECTIVE CONSCIOUSNESS) can know?
Is it possible any change in him without something different outside or OBJECTIVE CONSCIOUSNESS?
But, in the same time, is it possible to have OBJECTIVE CONSCIOUSNESS without SUBJECTIVE CONSCIOUSNES?
What is OBJECTIVE CONSCIOUSNESS if not many SUBJECTIVE CONSCIOUSNESS in interaction?
One without other can't exist. Therefore reality is always interaction of those two consciousness or opposites. SUBJECTIVE and OBJECTIVE CONSCIOUSNESS just have appearance of separation because they represent the same or physical existence in process of activity and change.