The only way I know for DRM ebooks is to install two different book reader programs.
You can install ADE2.0 and ADE1.7.2 and use them side by side.
Also Sony Reader (designed to use with Sony eReader devices but works ok just by itself) https://ebookstore.sony.com/download/
If they are not DRM there are lots more programs to choose between, and some of them can launch multiple copies of the same app if you prefer.
eg Cool Reader http://sourceforge.net/projects/crengine/
I don't know of any where a single instance can open two books side by side.
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-editio ns.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