if I already filed my taxes with turbo tax this year, how can I file another return for someone without getting my return mixed up with it?

For online TurboTax each taxpayer must have their own separate account and pay for their tax preparation software.  If you try to do two users on the same account you will overwrite things and lose your records.

with online create a new account with a unique user name. you can use the same e-mail address as yours. for desktop start a new return. save it using a unique file name. Turbotax usually supplies the taxpayer's name which is sufficient.