If you are using TurboTax Online, you can create multiple accounts for the same email address by using different usernames. When you go to log in once you have multiple usernames, you will see buttons for "Use a different userid" and "remove a userid". Each return will require separate payment for TurboTax.
If you use TurboTax Desktop, you can prepare up to five returns for one price. See here for more information about Desktop products.