
Actually it is possible the strongest ghost will take charge and the other one be dormant to the point it's like it never existed once one is removed the other will regain control.
The thing I don't understand is why kill his own brother? Did he thought that dying again in a new body would bring him peace?

So like Yoon Dam had explained on the bus about how a person can remember things the ghost was doing when their possessing someone. And he had used that dream example when explaining it to the brother who then said he has experienced dreams like that alot (which we now know probably was a foreshadowing of his brother taking him over months ago). So I think the brother realized how long things had been going on and couldn't cope since he didn't know why any of it was going on. Maybe even felt like he needed to follow his brother since it seems like his parents aren't around at all.
This is what I understood.
1) There are two ghost.
-Ghost 1: the student that died due to bullying
-Ghost 2: The bully that was in the army with MC
2) Both the Ghost possessed the bullied student's younger brother (poor kid)
3) MC exercised the 2nd Ghost.
4) The 1st Ghost still possessing the younger brother
What I don't understand is why is the 1st ghost trying to kill his own brother.