If you are using TurboTax Online, you'll need to create a new user ID and password (and pay again). It's better (and cheaper) to purchase TurboTax Desktop if you need to create multiple tax returns.
Only one return per taxpayer if using the online versions. Each must have a separate ID/password although you can use the same email address. To recover a previous account or get an emailed list of accounts tied to the same email address, see link below.
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Yet nowhere I can find description of steps how to file multiple returns on Desktop version. Unfortunately your software is the MOST user-UNFRIENDLY I ve seen in my life. I ve been using turbotax for a few years and this year is my last one. No more I will spend hours to find answers to the questions many people ask and talk between themselves trying to figure out how to move forward and your 'support' page never addresses any of these questions. This is not a free soft. You should follow up with answers to every question, not to mention questions asked by many people. I am stuck with turbotax this year, but looking forward to never deal with you again.