These two plots in the story line are connected because they are both part of Scout's childhood experience. The story is related as a narrative flashback; events are perceived through the eyes of a child but 'filtered' by the mind of an adult.
The two plots are also related in theme. Both deal with the topic of preconceived ideas towards ostracized people. In the adult world, the trial focuses on an innocent man who is convicted of rape simply because he is a Negro. (His word will not stand against that of a white woman.) Scout's, Jem's and Dill's fascination with Boo Radley portrays the natural curiosity of children but also a need to 'be scared' and then to overcome the source of that fear.
In the end, the children do a better job at this than the grownups. Whereas Tom Robinson is first convicted and then shot to death, Boo Radley, who was "on trial" in the children's own kind of 'kangaroo court,' becomes their hero and friend.
Both plots are similar in that they focus on upholding human dignity and fighting prejudice. Both Atticus and Bo are each brave in their fight for these principles. However, the plots show the different reactions to fighting for these principles.
Bo is an outcast of the town. He has made a choice not to appear in public. However, at the end of the novel, Bo makes a choice to fight for right in order to save the lives of the Finch children. This act of entering society brings Bo acceptance. Ironically, this is the opposite reaction Atticus gets when he decides to defend Tom Robinson. Atticus' fight for right doesn't make him more accepted by society. In fact is threatens to cut him off from society.
Yep there's two plots in the book and they deal with Boo Radley and Tom Robinson. It has to do with tom b/c of the trial and it has to do with boo b/c of how much jem, scout and dill are fascinated with him.
Think about the title, To Kill a Mockingbird, and remember Atticus' line where he reminds Scout and Jem that "it's a sin to kill a mockingbird." TKAM is all about prejudice and the differing forms, innocent (as with the children's perceptions of Arthur Radley) and ugly (witness the false accusation and ultimate death of Tom Robinson) that it can take. Arthur and Tom are both "mockingbirds" in that sense, because they are both kind, unassuming, and retiring - they bring nothing but pleasure to the people they know. Arthur's kindnesses toward the children are downplayed, but speak volumes about the true nature of this neighbourhood legend. Tom's kindness is toward Mayella Ewell. Because of their "otherness", however, both Tom and Arthur are viewed as dangerous, frightening. Arthur literally becomes "Boo", a boogie man, and Tom becomes that even more frightening figure, a black man with the audacity to show human kindness to a white woman. Thus the two men are connected, under the title, and their two stories share more than characters - they share the role of scapegoat for the ignorance and prejudice of this archetypal southern town.