Personally, I would choose two names from a piece of literature. That way the twins wouldn't be burdened with "rhyming names" or anything like that, but still connect in some way.
Examples:
Morgan and
Gwen (Arthurian legend)
Juliet and
Rosalind (
Romeo and
Juliet)
Tom and
Finn (The Adventures of
Tom Sawyer and Huckleberry
Finn)
Sara and
Emily (A Little
Princess)
Anna and Karenina (
Anna Karenina)