Chapters are individual Longform 'books'. Once you finish writing them, you can join them together.
When the first chapter of a Longform book is published there is a link underneath to begin the next chapter, and this joins the books together automatically.
If you haven't published the first chapter yet you can still write the next chapter and join them later.
You can use a different artist for each chapter for the interior art, but they will all share the cover image of Chapter 1.
When you're on your 'your stuff' screen you can join different chapters together into the same book. They will both need to be published before you can join them.
Find the second chapter of your longform book, and click on the gear icon just below the cover, and choose 'edit details'. Once you're on the edit details screen, choose the 'part of an existing book' option and then choose the 'first chapter' book from the list. Click on 'publish' at the bottom, and ta-da! - Chapter 2 has been added to your book.