multiple users on game center not working

I am sharing games with my wife on an ipod touch...In the past games like plants vs zombies offers the ability to switch users in the app. Now with game center it seems like game center is supposed to take care of that... At least Angry Birds and Infinity Sword have no option to switch users. I set up two accounts in game center but all of my accomplishments show up on her account and vice versa, anyone know how to fix this?