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The following is a list of elements I remember about this book series:
1) The cover art was very anime inspired, but was clearly an American or Canadian series.
2) Wyverns were involved. Not dragons, wyverns (dragons have forearms, wyverns have wings where their forearms should be).
3) The people in this world had the ability to control elements, provided that they had special stones implanted in their backs.
4) The main characters both had 6 of these stones implanted in their backs (most people could only afford 1 or 2 stones for their children).
5) The main characters were twins (a boy and a girl). The girl had the power to control earth, and I cannot recall what the boy could control.
6) The boy sucked at controlling his power.
This is all that I can remember. I must have read these books somewhere around 2002-2005, and they seemed reasonably new at the time.