جَاءَ الرَّجُلَانِ نَفْسَاهُمَا: dual emphasis is nominative with alif.
رَأَيْتُ الرَّجُلَيْنِ نَفْسَيْهِمَا: accusative dual with yāʾ.
مَرَرْتُ بِالرَّجُلَيْنِ نَفْسَيْهِمَا: genitive dual with yāʾ.
These are teaching examples. The referential pronoun humā stays the same; the dual case ending changes.
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Tawkīd: Verbal and Semantic Emphasis
جَاءَ الرَّجُلَانِ نَفْسَاهُمَا: dual emphasis is nominative with alif.
رَأَيْتُ الرَّجُلَيْنِ نَفْسَيْهِمَا: accusative dual with yāʾ.
مَرَرْتُ بِالرَّجُلَيْنِ نَفْسَيْهِمَا: genitive dual with yāʾ.
These are teaching examples. The referential pronoun humā stays the same; the dual case ending changes.