يَدَآ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – يَدَآ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

يَدَانِ is the natural state of this indefinite noun and it is in this state ( مَرْفُوْعٌ) when the noun is: 1) the subject المُبْتَدَأُ or 2) the predicate الخَبَرُ or 3) اسْمُ كَانَ or 4) خَبَرُ إِنَّ or 5) الفَاعِلُ subject of a verb in the active voice or 6) نَائِبُ الفَاعِلِ subject of a verb in the passive voice or 7) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

يَدَانِ loses it’s ن, changing to يَدَآ due to it being a possessed noun

Grammatical analysis of يَدَآ 
اسْمٌ مَرْفُوْعٌ وَعَلَامَةُ رَفْعِهِ الأَلِفُ لِأَنَّهُ المُثَنَّى وَهُوَ مُضَافٌ 
Surah/
Below are the 5 verses in the holy Quran containing the dual, possessed, feminine nounيَدَآ
Verse
The Jews said, “The hand of Allah (SWT) is chained”وَقَالَتِ اليَهُودُ يَدُ اللهِ مَغْلُولَةٌ
Chained are their hands and they are cursed for what they saidغُلَّتْ أَيْدِيهِمْ وَلُعِنُوا بِمَا قَالُوا
Nay, both His Hands are outstretched, He spends as He pleasesبَلْ يَدَاهُ مَبْسُوطَتَانِ يُنْفِقُ كَيْفَ يَشَآءُ
What has been revealed to you from your Lord, indeed increases many among them in rebellion and disbelieveوَلَيَزِيْدَنَّ كَثِيْرًا مِّنْهُمْ مَّآ أُنْزِلَ إِلَيْكَ مِنْ رَّبِّكَ طُغْيَانًا وَكُفْرًا
and we have cast among them enmity and hatred till the day of Resurrectionوَأَلْقَيْنَا بَيْنَهُمُ العَدَاوَةَ وَالبَغْضَآءَ إِلَى يَوْمِ القِيَامَةِ
Every time they kindled the fire of war, Allah (SWT) extinguished it and they strive in the earthكُلَّمَآ أَوقَدُوا نَارًا لِلْحَرْبِ أَطْفَأَهَا اللهُ وَيَسْعَوْنَ فِي الأَرْضِ
5-64spreading corruption and Allah (SWT) does not love the ones who corruptفَسَادًا وَاللهُ لَا يُحِبُّ المُفْسِدِيْنَ
18-57
22-10
Indeed, We warn you of a near punishmentإِنَّآ أَنْذَرْنَاكُمْ عَذَابًا قَرِيْبًا
the day when man will see what his hands have sent forthيَوْمَ يَنْظُرُ المَرْءُ مَا قَدَّمَتْ يَدَاهُ
78-40and the disbeliever will say, “woe to me! would that I be dust”وَيَقُوْلُ الكَافِرُ يَا لَيْتَنِي كُنْتُ تُرَابًا
111-1Perished, the two hands of Abu Lahab and perish heتَبَّتْ يَدَآ أَبِى لَهَبٍ وَّ تَبَّ