If I want to start another Turbo Tax Online return, how do I save my first return in my Documents?

you cannot have two returns for the same year in the same account. For a second return, you have to create a second account with a unique user ID. password is up to you. you can use the same e-mail as the first account. 

Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.  You can start another return in TurboTax Online by:

1. Logging out if you are in your account
2. Choose what TurboTax Online product you want to use for the second return
3. Create your account screen
4. Set up a new login for the second return
5. Start working on the new return

You can have up to 5 accounts can be tied to the same email address.