Why do you need 2 tax returns? Aren't you filing a Joint return? Online is only good for 1 return per account so you would need 2 accounts. If you do a second return in the same account it will overwrite the first one. If you need to do more than 1 return you should consider using the Desktop download program. It can do multiple returns and efile 5 federal for free. And you can use a lower version since all the Desktop programs have the same forms.
You can start another return in TurboTax Online by:
Logging out if you are in your account
Choose what TurboTax Online product you want to use for the second return
Create your account screen
Set up a new login for the second return
Start working on the new return