I like "Take Two: A Celebration of Twins" by Jane Yolen and J. Patrick Lewis. It's a PB collection of poems about twins, and includes fun facts on each page about twins, like the heaviest at birth, the oldest, difference between fraternal/identical, etc.
I bought it for my 12-year-old twin girls, and they enjoyed perusing it (even though they consider themselves past PB age), so I'm sure it would bring lasting reading pleasure to 6-year-olds.