The Grammarians by Cathleen Schine

The Grammarians started out great. Laurel and Daphne are red-headed identical twins who love words. It starts with their secret "twin language"--the translation of which is hilarious. And leads to careers as a writer and poet, respectively. But after being joined at the hip for years, they eventually drift apart then have a rift (over a dictionary no less) that lasts for years. Will they ever find their way back to each other? The reader doesn't find out till the very end, which was actually the best part of the book aside from the very beginning. It was the middle of the book that kind of bogged down, when the twins are in their snarkiest years.

I did love the twins' mother, Sally, who is never quite sure what to make of her young daughters but comes to understand them better than anyone.