ADE remembers the last position in every book you have ever read.
To switch between books you need to go back to Library view and choose the other book.
If you sort by last read (dropdown at top right) it is reasonably easy.
Might be even easier still if the 'Last Read' at top left included 3 or so books
(but considering all the other failings of ADE2.0 this is a fairly minor detail).
Thank You, but by asking my question I meant reading them simultaneously, that is side by side, in two windows for example. The problem is that I have a book in two different language versions and I would like to compare their content by reading them side by side...
Ah. As far as I know that isn't supported directly by ADE.
If the books aren't DRM you could use another ereader software for one book (or both books). For example CoolReader http://sourceforge.net/projects/crengine/
(If they aren't DRM, you would probably never used ADE in the first place.)
If the books are DRM you choice is more limited.
You could install both ADE2.0 and 1.7.2 and run those side by side, or install Sony Reader. https://ebookstore.sony.com/download/
Sony Reader can read Adobe DRM ebooks; it is designed to complement Sony eReader devices, but works find even if you don't have one.