Using the TurboTax online editions you can only file One tax return per account and User ID. To file another tax return using the TurboTax online editions you Must create a new account with a new User ID.
Using the TurboTax desktop CD/Download editions you can create, complete and file as many tax returns as needed without further charge after the purchase of the software.
Or do mean another tax form you got like a W2 or 1099? You have to report all your income on the same tax return. If you filed without something then post back and don't change or add anything yet.