One parent can clearly be abusive while another parent is a co-victim with the multiple.
Rane of Crisses probably put it best:
"Dad hurt us and mom neglected us; failed to protect us. I had to protect us. Funny: I don't think our father would have failed to protect us [from our other abusers, from the world], but he would have kept us in the dark and made us wither. Sometimes our mom got between us and our father, but I guarantee that he made her regret that. And our mom did other things that were bad...the [emotional blackmail] and borderline eating disorder stuff too."
When another parent is abused with you, you can be rest assured that they're going to need to defend themselves, and most people who remain in abusive situations are probably not acing the healthy defense mechanism test. Some abused parents will dissociate, deny, repress, or turn to medicating their problems with drugs or alcohol...and all of these can be harmful to a child, leading to neglect, or calling the entirety of the child's world into question. Many multiples probably lived with the duplicity of being abused and then being told it never happened --whether by the abuser or by other people. A child has a hard time dealing with this level of denial of reality, and as a result some psychologists believe they may split due to the dissonance in their reality.
There are also traumas involved in watching other people being abused. The passive parent takes damage when the child is abused, the child is damaged when watching the abused parent being abused. Specific types of helplessness affect siblings and other witnesses of abuse.
An example of this comes from Ariel of Deus Debris:
"According to my mother, my father was very verbally, emotionally (and very rarely, physically) abusive to her. Since this is something that 'rings true' but that I don't actively remember, it's very possibly something that caused me to dissociate."