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.

You need two accounts to prepare returns for 2 separate filers.

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 Account Settings (Profile and Security tabs), 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: