You can only do one return with the online TurboTax software. You have to open a separate account for her. With only W-2 income, she probably qualifies for the free edition.
If you need to do multiple returns and don't qualify for the free edition, consider using the down load version of TurboTax. There is no limit to how many returns you can prepare. But there is a 5 return limit for e-filing.
