Surya dev didn't sent his wife anywhere.
Sangya was one among Surya dev's wives. She was unable to bear the extreme radiance her husband had. So she created a superficial entity from her shadow called Chhaya and instructed her to take her place while she herself went to a mountain. Chhaya became the mother of two sons and two daughters.
As far as I know, Surya Dev was unaware of Sangya's chhaya staying as her. Later when he got to know it, he went to the mountain where Sangya was supposed to be. There only the Ashwini Kumaras were born. The twins got this name as their parents were in the form of horses on the mountain.