As I discuss in this question, one of the most prominent incarnations of Vishnu in ancient times was the sage Narayana, twin brother of the sage Nara and son of Yama god of death. Nara and Narayana were famous for engaging in Tapasya (deep meditation) in Badrikashrama, and in the time of the Mahabharata they were reborn as Arjuna and Krishna. In any case, apparently Narayana carried out a five-night (Pancharatra) Yagna and then became the entire universe, so people started following the so-called Pancharatra texts, scriptures originating from Narayana himself which gave detailed procedures for worshipping him. As I discuss in this question, the Pancharatra texts were important in the development of Vaishnavism. Among the oldest Pancharatra texts are the Satvata Samhita, the Paushkara Samhita, and the Jayakhya Samhita.
But there's a Pancharatra text even older than these, and it is found within the Mahabharata! Most people only know about the Bhagavad Gita, an 18-chapter religious discourse between Krishna and Arjuna found in the beginning of the Bhishma Parva of the Mahabharata. But there is another 18-chapter religious discourse found at the end of the Shanti Parva of the Mahabharata, the Narayaniya. It is a Pancharatra text which takes the form of a dialogue between Yudhishtira and Bhishma as the latter is lying on the battlefield of Kurukshetra, and Yudhishtira asks him questions about the importance of worshipping Narayana/Vishnu. (It goes from this chapter to this chapter in the Shanti Parva; apparently the Bhagavad Gita and the Narayaniya divide the Mahabharata into three equal parts.)
In any case, in the first chapter of the Narayaniya, Bhishma describes the birth of the sages Nara and Narayana:
I heard it from my sire that in the Krita age, O monarch, during the epoch of the Self-born Manu, the eternal Narayana, the Soul of the universe, took birth as the son of Dharma in a quadruple form, viz., as Nara, Narayana, Hari, and the Self-create Krishna. Amongst them all, Narayana and Nara underwent the severest austerities by repairing to the Himalayan retreat known by the name of Vadari, by riding on their golden cars.
My question is, what is the story of Nara and Narayana's brothers Hari and Krishna? The only other information the Narayaniya provides about them is Narada's statement that Hari and Krishna lived in Badrikashrama before Nara and Narayana:
That form took birth in four shapes for the expansion of the race of Dharma which have been reared by that deity. How wonderful it is that Dharma has thus been honoured by these four great deities viz., Nara, Narayana, and Hari and Krishna! In this spot Krishna and Hari dwelt formerly. The other two, however, viz., Nara and Narayana, are now dwelling here engaged in penances for the object of enhancing their merit.
The only other scriptural reference I could find to Hari and Krishna is in this excerpt from the Vamana Purana:
The divine body Dharma born from the heart of Brahma, had reproduced Hari, Krishna, Nara and Narayana sons by virtue of consummating with his wife Dakshayani.... Hari and Krishna engrossed themselves with exercise on Yoga. Nara and Narayana the greatest among all ancient sages began austerity with Parabrahma at the ever broad bank of the sacred Ganges in Badrikashrama, a place situated in the great Himalaya.
So first of all, how did Hari and Krishna's practicing of Yoga differ from their brothers Nara and Narayana's practicing of Tapasya? Second of all, what caused Hari and Krishna to leave Badrikashrama? Did they die or something? And in any case, what was Vishnu's purpose in incarnating as Hari and Krishna?
Are there any other Hindu scriptures which mention these two brothers? Note that this Hari is different from Vishnu's incarnation Hari who rescued Gajendra, whom I discuss here.