I know you're looking for distinctly matching names, but as a twin myself I can tell you the kids will probably hate it. I do like the sun and moon meanings, and Solarian and Lunaria individually, but what about more subtle names?
Sorin and
Aysu (sun and moon)
Cyrus and
Selene (sun and moon)
Horus and
Lilith (day and night)
Lior and
Isra (light and dark)
Lucius and
Cynthia (light and the moon)
Samson and
Lyra (sun and stars)