We are used to thinking of the moon and sun as opposites, a fallacy that even pervades astronomy, as Space.com points out. For a single instant in a lunar month (29.5 days) the sun is opposite the moon in the sky. This is the exact moment of the full moon. Apart from this, the two may be anything from 180 to 0 degrees apart, and are often seen in the sky together.
This may explain why Völuspá refers to the moon-god Mani as the sun’s companion or escort. (Depending on translation.) Also, when the two appear in the sky for the first time, their routes are not yet established, so it makes sense that they would be together.