In TurboTax Online you can only file one return per account (User ID) and would have to create a new User ID and password for each additional return created. You can, however, use the same email address for both accounts to receive notifications if you needed to. If you were to use the same account then the second return would overwrite the first. Below I have included a TurboTax FAQ on how to start a additional return in TurboTax Online but if you are interested in purchasing the CD/Download version of TurboTax then you would have 5 Federal E-files included and I have provided the link here as well for the website to purchase the CD/Download if you would rather that than the Online version.
TurboTax CD/Download link: