You have to create a new account for each return you want to file when using the Online version.  Each online account can only handle 1 Federal return.  If you try to prepare a second return in the same account, it will overwrite the first one, making it unrecoverable.

A separate account (separate User ID) is necessary for each Online return, and a separate fee (if applicable).   You can use the same email address in the Profile (account settings) if desired for notification purposes, but each User ID must be unique.

Please see this FAQ on how to start a new Online return:  

https://ttlc.intuit.com/replies/3288574