How does Harper Lee merge the two storylines of "To Kill a Mockingbird"?
The novel opens with a story of growing up in a small town. Jem and Scout have the normal adventures you would expect in this type of story. They come into conflict with their family, they encounter adventures with a rabid dog, an eccentric neighbor, and a young school teacher who clearly doesn't understand the culture of the town. They watch a man pee off the porch and then Jem and Dill have a peeing contest. This half of the novel is very similar to Tom Sawyer.
However, the novel becomes far more serious when the attempted rape of Mayella leads to the arrest of Tom Robinson. The second half of the book is far darker with threats of a lynch mob and serious conflicts between the morality of the town and Atticus Finch's beliefs.
A number ofdifferent events link the two. Jem breaks his arm, which is a very common small-town event that fits well within the parameters of the first story, and yet it is caused by Mr. Ewell which makes it part of the second story. Calpurnia takes them to church--a common event that should fit into the first story, and yet the discussions at the church of Tom Robinson and the need to support him make it part of the second story. The young Cunningham boy who comes to lunch after Scout's disasterous first day at school is part of the first story, and yet the Cunninghams play a major role in the second story both as members of the potential lynch mob and jury members. They are very closely linked.