Using the TurboTax online editions you can only complete one tax return per account and User ID. To start another tax return using the online editions you will need to create a new account with a new User ID.
https://turbotax.intuit.com/personal-taxes/compare/online/
If you used the TurboTax desktop CD/Download editions in the past, those editions can be used to complete as many tax returns as needed.
