You can't. Each online account can only handle 1 Federal return per tax year. If you try to prepare a second return in the same account, it will overwrite the first one, making it unrecoverable.
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 Account Settings (Profile), if desired for notification purposes,
but each User ID must be unique. A User ID does not have to be an email address.
For future reference, the desktop products (CD/download) can handle multiple returns.
Please see this FAQ on how to start a second Online return: