Online TurboTax can prepare only 1 Federal return per online account per year. To prepare a second return requires a different separate account (different User ID). You can use the same email address in the Account Settings for notification purposes in up to 5 accounts, if desired, but each log-in User ID must be unique.
Do NOT try to prepare a second return in the same account, or it will overwrite the first one.
For future reference, desktop editions can prepare multiple Federal returns with up to 5 Federal efiles.