Rule 1: 1. The dual in a sentence
Decide the case of a dual noun exactly as you would for a singular: subject or predicate, nominative ـَانِ; after a preposition or as the second word of an iḍāfa, genitive ـَيْنِ; object of a verb, accusative ـَيْنِ. Remember too that the comparative أَكْبَرُ does not change: it is the same after a masculine, a feminine or a dual noun.
مَعَهُمَا سَلَّتَانِ.
maʿahumā sallatāni
They (two) have two baskets with them.
لِلْبَائِعِ حَانُوتَانِ فِي السُّوقِ.
li-l-bāʾiʿi ḥānūtāni fi-s-sūqi
The seller has two shops in the market.
اِشْتَرَيْتُ بَيْضَتَيْنِ وَسَمَكَةً وَاحِدَةً.
ishtaraytu bayḍatayni wa-samakatan wāḥidatan
I bought two eggs and one fish.
ثَمَنُ الرُّمَّانَتَيْنِ دِرْهَمٌ.
thamanu-r-rummānatayni dirhamun
The price of the two pomegranates is a dirham.
هَاتَانِ السَّمَكَتَانِ أَكْبَرُ مِنْ سَمَكَتَيْكَ.
hātāni-s-samakatāni akbaru min samakatayka
These two fish are bigger than your two fish.
أَكْبَرُ stays unchanged after a feminine dual.