The TurboTax online editions can only be used for one tax return per account and User ID. Any returns completed using the same account after the first tax return will overwrite the previous tax return.
To use the TurboTax online editions for another tax return you would need to create a new account with a new User ID.
If you are going to do multiple tax returns you should use the TurboTax desktop CD/Download editions where you can create, complete and file as many returns as needed with only one desktop edition.
TurboTax desktop editions - https://turbotax.intuit.com/personal-taxes/cd-download/