You can use the same email address if you choose however, you would need to create a new login for the second tax return. Up to 5 accounts can be tied to the same email address.
If you're preparing multiple returns for family members, it's more economical to switch to the TurboTax CD/Download software, which lets you e-file up to 5 federal returns at no additional cost. More info
Please follow the steps in the FAQ for instructions on starting another return in TurboTax Online.