The Amy Sutton from Sweet Valley Twins and the one from Sweet Valley high are different people.The books have a lot of girls that look just like the Wakefield Twins and pretend to be them on occasion (Margo being the most notable example). Who says that only Elizabeth and Jessica get doppelgangers? That could explain why Amy underwent such a drastic change between the two series. In Sweet Valley Twins, she's a smart and lovable tomboy and the loyal best friend to Elizabeth. In Sweet Valley High, she's a shallow, boy crazy bitch. Maybe somebody (Margo?) found a girl who looked like Amy and sent her to Sweet Valley to spy on and take down the Wakefield twins. It didn't work because the replacement wasn't very smart and was more interested in hooking up with hot Sweet Valley guys. Ken's car accident in "That Fateful Night" wasn't actually an accident. The mastermind warned Faux-Amy to stay away from Real-Amy's middle school boyfriend because he might get suspicious. She couldn't resist the hunky football star so the mastermind arranged for him to get into a car accident in hopes of getting him out of the way. It worked when Faux-Amy lost interest in Ken as a result of his blindness. The scheme is discovered after Sweet Valley High. Faux-Amy is sent to jail while the real Amy is freed and decides to stay far, far away from Sweet Valley and the Wakefield twins (explaining why she wasn't in Sweet Valley Confidential).
The current Sweet Valley High series "The Sweet Life" is a massive attempt at damage control after the backlash over "Sweet Valley Confidential"