You don't. In TurboTax online each return must be done in a unique account. You will need to create a new account for the second return.
In TurboTax Desktop (from a disc or download), you can create multiple tax returns.
I always purchase the download edition which lets you e-file 5 tax returns for free (and 1 state -additional states cost $45) and ends up being cheaper than the online option. I inevitably end up 'helping' one of my 20-something kids or other relatives file their taxes (this year 2 kids and my partner's aunt). It is impossible to find the download edition on the TurboTax website - you need to specifically search for it. Google 'Turbo tax CD/download' I realize this does not answer your question - but is how I solve for the problem.
worthless answer,
no help , please answer the question for TT desktop
With the Desktop software, you just go to the upper left corner
click on <File
to get the dropdown menu ....then select
<New Tax Return
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IF using the Desktop software, it's easy...but you should also check what's in all those top-menu selections so that you familiarize yourself as to what's available in the future. New menu items will show up once you actually get a new tax file started.
Make sure you get your own taxes prepared and filed before you do anyone else's. If doing a child's tax return and that child has a similar name to yours, make sure you select Save-As, and give that tax file a unique filename, so it doesn't overwrite your own.