I have a friend who married a man who's wife committed adultery as did he. There is a question of who committed adultery first. I told her I do not think it matters who committed adultery first and that her marriage may not be valid because of the adultery in the past marriage.
Because there is a question about who was unfaithful first, it appears that both parties were guilty and that the divorce was over each other's unfaithfulness. In this case there is no innocent party and neither has the right to marry again since each can say they divorce the other because of their adultery (Matthew 19:9).