If you are using Online TurboTax, it can only prepare 1 Federal return per online account per year. Do not attempt to prepare a second return in the same account, or it will overwrite the first return, making it unrecoverable. It is the desktop product where one can prepare multiple Federal returns and efile up to 5.
To prepare a second return in Online TurboTax requires a different account with a different User ID. You can, however, use the same email in the account profile for notifications for up to 5 accounts, but each User ID (log in) has to be unique.