Also impossible to have two accounts open at the same time even on ADE, let alone standalone ereader devices.
The only way I know round this is to use a DRM stripper such as epubee.
Do this for the books on the account you want to lose while you still have a computer authorized with this account.
I do NOT advocate using such code to get around the valid limitations placed on a DRM book.
However, where the problem arises only because of the ineptness of Adobe's ADEPT DRM infrastructure and its implementation,
such a measure is totally justifiable.
I see that I was wrong: it is possible to merge accounts (as long as you have id and password for both)
How do I find out which accounts are joined together?
You can go to this page: https://adeactivate.adobe.com/adept/en/JoinAccountLoginForm.jsp to see
which accounts are joined together.