The name of the series is "When She Was Bad", implying that at whatever future date the story's being told at she's not anymore.
That's why the bolt of energy that changed them both passed through Amber on its way to Gail. Amber got most of the powers, but Gail is the savior the mystics were looking for. For all her faults, Gail is loyal and takes care of the people she cares about. Alternately, Amber and Gail are the chosen Two
and need to work together to save the world, even if they hate each other