assalaamu alaikum.
Scholars said that من in مِن قَلْبَيْنِ is من الزائِدَةُ. Here is some explanation about it from books which I have read:
A Glossary of words used in Durus al-lughat al-arabiyya li ghair al-natiqina biha
مِن الزائِدةُ,the word مِن used with a نكرة in a sentence containing negation, prohibition or interrogation. It signifies emphasis, e.g., ما جاءَ مِن أحَدٍ, no one has come. لا يَخْرُجْ مِن أَحَدٍ, no one should leave. هَل مِن خَبَرٍ؟, any news?
KEY TO Durus al-lughat al-arabiyya li ghair al-natiqina biha, PART III
هل مِن سؤالٍ؟ 'Any question?'. The full construction of this sentence is like this: هل مِن سؤالٍ عندك؟ 'Do you have any question?' Here سؤالٍ is mubtada' and عندك is khabar, and مِن in this construction is called مِن الزائِدةُ (the extra min), and is used to emphasize the meaning of the sentence. There are two conditions for using the extra min. These are:
1) The sentence should contain negation, prohibition or interrogation. The interrogation should only be with the particle هَلْ.
2) The noun following the extra min should be indefininte, e.g.,
Negation : ما غَابَ مِن أحَدٍ 'No one is absent', ما رأيتُ مِن أحَدٍ 'I did not see any one'.
Prohibition : لا يَخْرُجْ مِن أحدٍ 'None should go out', لا تَكْتُبْ مِن شَيءٍ 'Don't write anything'.
Interrogation : هَلْ مِن سُؤالٍ؟ 'Any question?', هَلْ مِن جديدٍ 'Anything new?'. In the Quran (50:30) : يَوْمَ نَقُولُ لِجَهَنَّمَ هَلِ امْتَلَأْتِ وَتَقُولُ هَلْ مِن مَّزِيدٍ 'On the day when We will say to Hell, "Are you full?" and it will say, "Have you any more?".'
Note that the noun following the extra min is majruur because of this مِنْ, and loses its original ending, e.g., in ما رأيْتُ أحدًا the word أحدا is mansuub because it is مفعول به; but after the introduction of the extra min it loses its nasb-ending and takes the jarr-ending even though its function remains what it was before. In the same way, in ما حضَرَ أحَدٌ the word أحدٌ is marfuu' because it is فاعل. After the introduction of the extra min أحد becomes majruur, though it remains فاعل in the sentence.