... a copy of part of that work, as it appears in that edition, shall be taken to contain only a reasonable portion of that work if the pages that are copied in the edition…exceed, in the aggregate, 10% of the number of pages in that edition but contain only the whole or part of a single chapter of the work

That makes it sound like they could copy the whole of the larger of two chapters of a two chapter book, even if that chapter were 90% of the book.