Reunited

I like to picture that Celestia and Luna fighted Discord on their own for most of his time as ruler of Equestria and only at the end they managed to find each other and defeat the mad spirit.
After they triumphed and cleansed the land with the Elements, the sky stayed still in a perpetual twilight with both the Sun and the Moon visible thorough the heavens, until, combining their powers, they took charge of the celestials orbs and set the cycle back in motion, granting Equestria peace and stability for centauries to come.