Maitravaruna is a term for Sage Agastya and Sage Vasistha in Hinduism as they were born from a single pot. Mitra and Varuna are two different celestial. When joined, these two names become Mitravarunau (dual form). The offspring of these gods is called Maitravaruna.
According to Devi Bhagavata (Skandha VI), King Nimi of the Ikshvaku dynasty wanted to perform a yajna with Vasistha as the chief priest. The yajna was to last for 500 years. Vasistha, who had already to the request of Indra to be the chief priest at a similar yajna, asked the king to postpone the ritual. The king, however, had his own way. He got the yajna performed with Gautama as the priest. This ritual, too, lasted five hundred years.
Vasistha returned to the palace of Nimi after completing the ritual of Indra. He came to know that the king had already completed the yajna. He was made to wait for the king who was asleep. Vasistha felt insulted and cursed Nimi that he would be bodiless. The powerful Nimi laid a similar curse on Vasistha in return.
A worried Vasistha then approached Brahma, who advised him to go to the ashram of Mitra and Varuna and stay there for some time, and merge with their effulgence. Brahma told him that, after some time, he would be born by himself without being in a womb.
Accordingly, Vasistha went to the ashrama of Mitra and Varuna and merged himself with their effulgence. One day the celestial nymph Urvashi came to their ashrama. Overpowered by her exquisite charm, Mitra and Varuna got excited. Their semen fell into a jar from which Agastya and Vasistha were born.
Since Agastya was born out of a pot, he is also known as Kumbhajanma or Kumbha Sambhava (one who was born out of a pot).
Puranas mention three births of Vasistha – the first from breath of Brahma (prana) and the second from yajna of Brahma. In the third birth Vasistha was born form a pot as the son of Mitra – Varuna.