This is a great question. Harper Lee's novel To Kill a Mockingbird definitely has two main story lines that are connected through Atticus' role as defense attorney. His defense of Tom Robinson -- or, more specifically, his questioning of the Ewells while they're on the witness stand -- is what brings about Bob Ewell's attack on his children.
The two climaxes are different in a number of ways. Tom Robinson is probably innocent (I'm not sure that we can ever be certain, but we're certainly led as readers to side with him at the trial) and Ewell is probably not. Robinson's judgement happens in the public eye, in a supposedly rational and unbiased manner (the law is supposed to be the great equalizer). Bob Ewell's attack and the subsequent cover-up of Arthur Radley's probable murder of Ewell by the sheriff happen at night and with only a very few people involved.
Depending on how you define the terms, you may see the first climax as unjust and the second as just. I tend not to see the cover-up as real or true justice, at least no more real or true than the Robinson trial, but I think many readers (and teachers) of the novel would tend to disagree with me. One of the strengths of this novel, I think, is that it can open up all sorts of interesting discussions.
In Harper Lee's book "To Kill a Mockingbird" the two experiences, the trial ending and Halloween night build to a climax with some significant differences.
The trial takes place in the town center at the courthouse. People from far away come to the courthouse. The Black people and The White people are segregated in the courtroom. Scout and Jem sit in the section with the black people. The tension builds as the children wait with the others in dead silence. The clock strikes every few hours but no one comes out with the verdict. The children are sure that Tom has been proven innocent. The room is hot but everyone feels cold. When the jury comes out they give a slip of paper to the judge. Three times the judge states, Guilty! Atticus gets his bags and walks out. The black people stand out of respect to him and Scout and Jem are told to stand as well.
The scene after the program at the auditorium is desolate. It is night and no one is around. Scout can barely see because she is in a ham costume. It is a hot night. The children hear a noise and they begin to get nervous. They are attacked by someone. Scout takes off and sees a man carrying Jem to her house.
The diferences are:
A) The courthouse is packed with people but it is silent.
B) The night the kids are walking is silent but they hear noises.
A) There are people all over during the trial.
B) Jem and Scout start out alone, but there end up 4 people in the woods.
A) Three times the judge calls out, "Guilty."
B) Three times Jem yells for Scout to, Run!
A) The mockingbird is Tom and he is subjected to guilt even thought he is innocent.
B) The mockingbird is Boo Radley and he kills Mr. Ewell while protecting the children and no charges are brought against him.
A) The events while the people await the verdict unfold slowly.
B) The events of the walk home and attack happen quickly.