Joseph and Gordon are similar because they are both leaders, and both have extreme ideas.
Joseph Strom is a violent and intractable man. David describes him as a “stern, implacable” man (Ch. 3). He is so difficult that his half-brother Angus describes him as “flinty-souled pedant, and bigoted well beyond reason” (Ch. 4). His eye is always open for anything slightly unusual, and he is quick to proclaim it blasphemy and destroy it.
It was because my father was a careful and pious man with a keen eye for an Offence that we used to have more slaughterings and burnings than anyone else… (Ch. 2)
The problem is that Joseph is quick to brutally beat his son or turn in a little girl like Sophie, all in the name of piety. He seems to lack tenderness or common sense. Everything is extreme with him.
When David escapes to the Fringes, he meets his uncle Gordon, who is the leader there. At first, he calls him the “spider-man” because of his long arms. It was because of those long arms that he was sent away at four years old.
Gordon is harsh, having been sent away at a young age. Unlike Joseph, Gordon seems to have a softer side, Sophie explains to David.
He's kind to me, David. He's fond of me. You've got to have as little as I have to know how much that means. You've never known loneliness. You can't understand the awful emptiness that's waiting all round us here. (Ch. 15)
Although both men are violent, they have different reasons for being so. David’s father prides himself on being perfect, and keeping a perfect public image. Gordon just fights to survive. He was thrown out at a young age, and that was the foundation for the rest of his life. As the firstborn son, he would have inherited everything. Instead, his arms were deemed too long and he was sent away. On the Fringes violence was a way of life, and the only way to survive was to be tough.
In these two brothers, David sees two sides of himself. He might become an intractable despot like his father, or a violent renegade like his uncle. He is different, and there is no escaping that. He can either try to join the outsiders on the Fringes, or cooperate with the Sealanders and leave the world he knows behind.