What is the role of «اللَّتَيْنِ» in «رَأَيْتُ اللَّتَيْنِ»?

Relative Nouns — Ṣilah, ʿĀʾid, Forms and Syntax

• Nominative dual subject
• Nominative predicate
• Accusative dual object marked by yāʾ
• A preposition