face time on 2 ipods

Both my kids have iPod touch 4. The e-mail address is the same for both, when I set up facetime my daughter would get calles from her friends and my son would get calls from his friends. I synced the 2 iPods and now when 1 of them gets a facetime call both iPods ring. How do I correct this and keep the password the same on both for purchasing items? Any help would be greatly appreciated.