You don’t. Each tax return filed using TurboTax online MUST come from a separate account with a unique log in name.
I don’t what you did in the past, but if you used one TurboTax online account to file two returns in the past, it is likely the second return was never actually filed.
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.