Two of the fairest stars in all the heaven,
Having some business, do entreat her eyes
To twinkle in their spheres till they return.
i.e., Two of the fairest stars in heaven, who have to leave their orbits, ask Juliet's eyes to twinkle in their place until they return.
Elizabethans believed Ptolemy's theory that the Earth was the center of the universe and that celestial bodies moved around the Earth in their crystalline spheres (or orbits). Fixed stars were last in the order of the spheres.
Shakespeare uses similar imagery in Hamlet (1.5.22) and King John (5.7.79), and, in Troilus and Cressida, he illustrates his knowledge of the Ptolemaic Universe:
The heavens themselves, the planets and this centre
Observe degree, priority and place,
Insisture, course, proportion, season, form,
Office and custom, in all line of order.