I belong to two libraries: Library A and Library B. Digital Editions associated itself with my library account at Library A. When I look at my "Computer Authorization", it lists the number on my library card for Library A, and it says that this is my "default account." That would seem to suggest that I can add other accounts, such as that for Library B, but I don't know how to go about doing that.
Yes, you can add multiple accounts to ADE,
1. Open a book that is fulfilled with a different Adobe ID account, ADE will prompt for the message that the book needs to be authorized.
2 Click Continue, you will be taken to the authorization dialog
3. Enter the credentials of other id [with which book was fulfilled]
4. Now ADE will be activated with both the id's.
Hope this works !!