Iwasaki's formulation is unclear. What he means is that besides the one NP in a relative clause that is made into a relative pronoun (or ortherwise marked as being relativized), you cannot relativize a second NP in the same relative clause. For instance, relativizing "the book" in "John gave the book to the girl" gives the relative clause "which John gave to the girl". If you then try to relativize "the girl" in this relative clause, you get the unacceptable *"who(m) ... which John gave to".
For instance, *"the girl who(m) I read the book which John gave to" has had two relative pronouns removed from the clause "John gave  to ". Evidently this is prohibited. John Ross in Constraints on Variables in Syntax studied such cases, compared the English examples with Japanese examples, and concluded that the Complex NP Constraint, as he dubbed it, applies only to "chopping rules" and not to "copying rules". With chopping rules, nothing is left in the original position of a relative pronoun, while something may be left behind by a copying rule.