You can only file one return per account with Turbotax Online, so you will need to create a second account. Sign out of your current account, return to the intuit.com and select Sign In. Then select the blue Create an account at the bottom of the screen (see screenshot below).
Tip: 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. Here is the link to the Desktop products: https://turbotax.intuit.com/personal-taxes/cd-download/