The Online version can only prepare 1 Federal return per account per tax year. The desktop version has more features than the online version. If you were used to the desktop version, you'll probably also miss Forms Mode, which the Online version does not have. The desktop version also does not have any form limitations like some Online editions do.
After you read the answer below, you may want to switch to the CD/download version for the other returns, unless you can use the Free Edition for simple 1040A or 1040EZ returns.
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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.
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), 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:
https://ttlc.intuit.com/replies/3288574
