When you use Online TurboTax, you get one return per fee. Each return needs its own account and user ID. If you use the same account and user ID for a second return, you overwrite the first one and lose it forever.
When you use Desktop TurboTax, which you install from a CD or download to your own hard drive, you can prepare and e-file multiple returns for the cost of the software. You can e-file up to 5 Federal returns—more if you mail the additional ones. If you have multiple returns to prepare for family members, Desktop is more cost effective.