Holly Goldberg Sloan & Meg Wolitzer 
To Night Owl, From Dogfish [EPUB ebook] 

Told almost entirely through emails, this is the exuberant, funny story of two girls who try to get their gay fathers back together. Its a novel about the true meaning of family, written by two exceptional and beloved authors. Avery Bloom is anxious and academic and afraid of the water. Bett Devlin is brash and athletic and loves to surf. The only things they seem to have in common are their agetwelveand the fact that their dads have fallen in love with each other. Now their dads are sending them to the same sleepaway camp, against their will, so that they can become friends. But when the girls reluctantly grow to like each other and start looking forward to becoming a family, their dads fall out of love. Can Avery and Bett figure out a way to bring their fathers back together now that they cant imagine a life without a stepsister?