Most people know about Radha, who is said to have been Krishna's lover and his favorite among the Gopis, whom Krishna spent time with during his childhood in Vrindavan and Gokulam. Yet the Sri Vaishnava sect (of which I'm a member) doesn't emphasize Radha. In part that's because Sri Vaishnavas focus on the Vishnu Purana and the Srimad Bhagavatam, neither of which even mention Radha's name. But as I discuss in this question, Sri Vaishnavas do recognize a figure similar to Radha, known as Nappinnai.
The story of Nappinnai is mentioned in a variety of scriptures, including the Harivamsa, the Garuda Purana, and the poems of the Alwars. Nappinnai was the daughter of Yashoda's brother Kumbhaka and the incarnation of Vishnu's third wife Niladevi (whom I discuss here). Krishna married Nappinnai after defeating the seven ferocious bulls of Kumbhaka.
Now this story bears a striking resemblance to the story of Krishna's queen Satya. As I discuss in this answer, Krishna is said to have married Satya after defeating the seven bulls of Satya's father Nagnajit, king of Ayodhya. I had previously assumed that this resemblance was due to some confusion in how Hindu scripture was transmitted; perhaps Satya was based on Nappinnai or vice versa. But then I came across this chapter of the Garuda Purana, which says this:
The maiden ... was born in the house of Kumbhaka and was called Nila. Kumbhaka was the brother-in-law of Nandashobha. He was the first who carried Kavya to the Pitris. He was abiding on the mount Vrishabha, at that time. I went to his house, O best of birds. Being unconquerable, by the boon conferred by lord Shiva, I killed him and was married to Nila.
In her second birth, Nila was born as the daughter of Nagnijit, Kavyavaha. In the Swayamvara of Nila, I controlled seven bulls who by the favour of lord Shiva were uncontrollable by gods and mortals. I conquered kings who had assembled at the ceremony I married her. Thus Nila was born twice on earth and married to me.
So my question is, what is the relation between Kumbhaka's daughter Nappinnai and Nagnajit's daughter Satya? Was it that Nappinnai died and was reborn as Satya? Or did these two incarnations of Niladevi live side-by-side as wives of Krishna?
Here is what Madhvacharya says in this excerpt from his Mahabharata Tatparya Nirnaya:
Later Shri Krishna left to the residence of Nagnajit, and in the swayamvara restrained seven oxen. They were oxen with the strength of daemons due to the boon of Shiva and could not be restrained by anyone else. All the other kings were defeated by them. Later Shri Krishna married Nagnajit's daughter Niladevi. Another maiden by name of Nila who was cowherd earlier entered her body. One person had incarnated in two forms.
So what does it mean to say that Nappinnai "earlier entered [the] body" of Satya? Did they merge when both were alive, or did Nappinnai die and her soul go to Satya, or was Nappinnai's soul also born in Satya's body while Naappinnai was alive the whole time?
Are there any scriptures that discuss what happened to Nappinnai in later years?