رَأَيْتُ اثْنَيْ عَشَرَ طَالِبًا — I saw twelve students.
رَأَيْتُ: past verb plus the subject pronoun.
اثْنَيْ: accusative direct object marked by yāʾ on the dual pattern.
عَشَرَ: second part, fixed on fatḥ.
طَالِبًا: accusative tamyīz.
The shift اثْنَا → اثْنَيْ follows the governing syntax; it is not an arbitrary alternate spelling.