The earliest firmly dated occurrence of any of these divine names is not in India and is not in a religious text. A treaty between a Hittite king and a ruler of Mitanni, preserved in the Hittite archives and belonging to the fourteenth century BCE, calls as witnesses on Mitra, Varuna, Indra and the Nasatyas, in a form of Indo-Aryan, in that order. The Rigveda cannot supply a date for itself. A diplomatic document in northern Syria can, and it puts the twins under their older name into the record at a fixed point.
That is one half of what makes them interesting to comparison. The other half is that divine twins associated with horses, with rescue at sea and with the dawn recur across the Indo-European languages, in the Greek Dioskouroi, in the Baltic material, and in scattered traces elsewhere. The correspondence is one of the stronger cases in the comparative study of Indo-European religion, and it does not depend on the etymology of any single name.
What the twins originally stood for, if they stood for anything, is a different matter and it has never been settled. The two twilights, the morning and the evening star, the sun and the moon, day and night, and a pair of stars have all been proposed. The disagreement is old: Indian commentators were already recording rival opinions about the Ashvins long before any European scholar took the question up, which is unusual and tells you the obscurity is not a modern problem. The atlas holds the question as argued rather than open, because the positions are real and can be read, and it does not adopt one of them.