Using the TurboTax online editions you can only complete one tax return per account and User ID. To start a second tax return with the online editions you would have to create a new account with a new User ID -
https://turbotax.intuit.com/personal-taxes/online/
Using the TurboTax desktop CD/Download editions you can create, complete and file as many tax returns as needed -
https://turbotax.intuit.com/personal-taxes/cd-download/
