With TurboTax Online you have to use a separate user ID, and pay separately, for each return. You cannot prepare another tax return with the same user ID that you used for your own return.
See the FAQ at the following link for instructions to start another tax return with a new user ID.
https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online
When you have to file multiple returns, it's usually less expensive to use the CD or download TurboTax software. You pay for the software once and you can use it for multiple returns. There is an additional charge for each state e-file, but it still usually works out to be less expensive. The CD or download software also has features that are not available in TurboTax Online. The most significant feature is forms mode, which lets you look at the forms while you are working on the tax return, and even make entries directly on the forms.