Is it possible to open several documents simultaneously in Adobe Digital Editions?

As far as I know, it isn't.

You can run both ADE2.0 and ADE 1.7.2 at the same time, with different books open.

Also Sony Reader (free download intended for Sony eReader users, but works even without) will let you have a third book open.

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There are lots of bugs in ADE2.0.  Try replacing ADE2.0 with the older but more reliable v1.7.2.

(You can have them both installed at once if you like.)

Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.

http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editions.html

The forum software is sometimes corrupting the link above.  There shouldn't be a blank in 'editio ns.html'.  The following redirects to the same page: http://tinyurl.com/diged172

https://ebookstore.sony.com/download/