غُلَامَانِ — غُلَامَا زَيْدٍ
Both expressions refer to two servants.
In the second, nūn is deleted because of iḍāfah, not because the noun ceased to be dual.
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Dropped Nūn Does Not Mean Singular
The Dual Noun: Inflection, Maqṣūr, Mamdūd and Iḍāfah
غُلَامَانِ — غُلَامَا زَيْدٍ
Both expressions refer to two servants.
In the second, nūn is deleted because of iḍāfah, not because the noun ceased to be dual.