While Diamond spends much of Chapter 18 talking about the differences between American societies and Eurasian societies, he does not actually devote any chapter or large portion of the book to specifically discussing differences between North and South America. Instead, we can only see scattered references to ways in which these areas differed.
We can see, for example, in Chapter 14 that states existed in South America (the Inca) where they did not really exist in North America before contact. This is repeated at the end of Chapter 18. This is why we read about the Spanish conquest of the Inca in Chapter 3. The Inca (and the Aztecs) had states where North American peoples largely did not. We are told in Chapter 5 that food production arose in South America about 1,000 years before it arose in Eastern North America. But these are simply factoids about the two continents. There is no detailed comparison of North American and South American societies in this book.