To refer to "the first and second chapters", do I say:
capitula prima et secunda
capitula primum et secundum?
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It appears that the noun can be singular or plural but the ordinals should be singular. That is, you'd need capitulum or capitula with primum et secundum. If you go with capitula prima et secunda, it sounds like there are several first chapters and several second chapters as in a collection of books. (At least I found no evidence to support this possibility to express what you want.)
Two Adjectives in the Singular may limit a noun in the Plural, as; prīma et vīcēsima legiōnēs, the first and twentieth legions.
The example in the Grammar seems to be lifted from Tacitus:
Primam ac vicesimam legiones Caecina legatus in civitatem Vbiorum reduxit…
However, it is also possible for the noun to be singular:
Primam et secundam aciem in armis esse…
(Caesar, De Bello Gallico, 1.49)