Book #32 in the series Animorphs by K.A. Applegate.    Animorphs
#32 by K.A. Applegate  Summarized Plot:  While morphing a
starfish to get an earring out of a tide pool crack, Rachel gets
cut in half by a mischievous boy and ends up becoming two
Rachels. But the two halves of her are also polarized: a mall-
crawling, airheaded, cowardly one with a sense of duty and an
ability to plan, and a psychotic, violent, courageous one with
an ability to attack and no decent foresight or memory. The
Animorphs are trying to destroy an Anti-Morphing Ray built by
the Yeerks, but their issues trying to deal with a Rachel too
afraid to fight and a Rachel too out-of-control to take orders
end up overshadowing the mission. But soon, through each of
their failures to function as a whole person, they realize they
need each other as much as they hate each other, and Ax and Erek
come up with a way to blend them together again . . . as long as
they can be convinced to carry it out and accept their opposite
as part of themselves agai