Roger Middleton and Dodger Cheswich areey two kids growing up on opposite sides of the country, Roger in Cambridge, MA, and Dodger in Palo Alto, California. They're both adopted, the same age, and perfectly ordinary kids.
Except they're not.
Roger's parents and teachers are impressed by his quick grasp of and love for English, but frustrated by his struggles with math. Dodger's parents and teachers are equally impressed by how easily she masters math, and frustrated by her struggles in English class.
Eventually, they both seemingly start to master their weak subjects. What's really happening is that they have started communicating in their heads, not quite telepathically but close, and coaching each other.