abuse on both sides is very uncommon. most often, the behavior of the victim, which can become less stable than that of the abuser as a result of the abuse, might be interpreted as abuse due to the PTSD/C-PTSD that occurs among DV victims, along with possible substance use/abuse. DV is comparable to being a prisoner of war. You cannot expect "normal" behavior by its victims. some may have stockholm syndrome. some may be highly reactive and irate - always on a hair trigger/fight/flight reaponse. some may be passive and terrified. some may stand up for themselves to the degree it appears as abuse, despite the fact their pleas fall on deaf/dead ears. many DV victims do not realize their partner is abusive. some just do not understand why their partner never listens, doesn't care, doesn't stop doing what the victim fears. they think their partner just doesn't understand, or is kind of emotionally immature, or is over-stressed, or overworked. they have no idea their partner does not care and enjoys tormenting them - enjoys their misery and feeds on it.
My MS in psych focused on psychopathy and PTSD and how they are opposites, but how the amygdala in the abuser and victim become mirrors of each other. this is why it can be hard to tell who the abuser is. the victim acts as a surrogate for the emotions that the abuser cannot acknowledge or manage. the abuser inflicts his emotions (shame, fear) onto the victim for her to live them out and manage them for him. the victim either takes on the unconscious trauma of the abuser and becomes traumatized and an abuse victim, or transmutes the trauma into healing (rarely unless she gets out and gets treatment). but the abuser feeds on the self doubt, fear, anger, depression, low self esteem of the victim - she makes him feel like he is a better, stronger person than she is. if she gets angry, he stays calm and baits her, feeling superior and calm when she is irate and he is chuckling on the inside. considering she usually was a very strong, talented, smart person to begin with, this makes his feel very powerful. he has the power to destroy her and dump his trauma onto her by re-enacting his own suppressed traumas on her and taking the superior role. he plays her like a marionette, and she thinks she is in an equal, human relationship. it is very confusing.