In Wild Seed, what did the characters Nweke and Thomas represent for Doro and Anyanwu, respectively? What made each so important? Why does Octavia Butler present them side by side (so to speak) in the narrative, forcing the readers to compare them?

Juxtaposing good and evil, male and female, power and weakness, Octavia Butler accentuates the gray area between these binaries. Doro seems totally reprehensible while Anyanwu attracts the reader's sympathy, but throughout the novel their roles overlap further. Their respective relationships with Thomas—and the resulting child who reveals the falws in the supperace theory—work towards changing them.
Doro is immortal and feeds his life through the bodies of others. To help breed more creatures to further his superrace of exceptionally powerful beings, Doro compels the brutal Thomas to have sex with Anyanwu. Their daughter is Nweke. The resentful Doro later takes over Thomas's body.
Anyanwu, the titular wild seed, is also immortal; her gifts including shape-shifting and healing. Her transformations include males and females. Seeking protection and security in the New World, she succumbs to Doro's manipulation and temporarily endorses his idea of creating the superrace. Her relationship with Thomas generates her compassion for him but turns her further away from Doro.
Thomas, whose illness helps weaken his resistance to Doro's domination, comes to care for Anyanwu because she heals him. Nweke not only inherits Anyanwu's gifts but has even greater powers. Unable to exercise sufficient mental or physical control, she suffers a breakdown. When she attacks Isaac, he instead kills her.