The TurboTax online edition can only be used to complete and file one tax return per account and User ID. To create, complete and file another tax return using the TurboTax online editions you will need to create a New account with a New User ID - https://turbotax.intuit.com/personal-taxes/online/
You can have up to 5 User ID's associated with one email address.
If you want to complete and file multiple tax returns, then you should be using the TurboTax desktop CD/Download editions - https://turbotax.intuit.com/personal-taxes/cd-download/