The alternating POV provides a balance that would otherwise not be present if only one or the other girls were the POV characters. Having the two, as many of you have pointed out offers a moderation of extremes. While it is clear that Emma-Jean is the subject and protagonist, with most of the narrative from her POV, I still wonder why add Colleen.
I wonder where/if Emma-Jean is on the Autistic Spectrum. Her behavior and thoughts indicate that she is not just “strange” but that there might be something wrong–her inability to communicate with peers, unable to empathize emotionally, priding herself on being strictly logical, incredibly gifted, yet connects more with adults than peers. Had Emma-Jean been the only POV, we would be unable to see the contrast with her 7th grade peers as deeply and affectively. There are moments when we ask “Oh, no Emma-Jean! WHAT ARE YOU DOING?!” because consequences of her actions are not obvious or logical to her.
The third person over the first person allows us to sympathize with EJ as well as other characters; a first person might bring us too close to EJ, allowing our sympathies to become TOO closely aligned with the character, accept her as a gifted anomaly and not question her actions–which, although EJ justifies them, are not only illegal, but the prank with Laura is cruel.
So, the need to contrast the two is crucial. EJ, from a different POV might paint her as “evil” or plotting, when her intentions are good, though misguided. Colleen offers some of that, while we also see into EJ’s mind that she is as good as Colleen, and sympathetic, but in very different and contrasting ways.
Overall I adored this story and you should read it.