How can I view two files side by side in Notepad++?

In Notepad++ 5.7 you can right-click on a tab and select "Move to Other View". You can also select "Clone to Other View" if you want to have the same file open more than once and see different parts of it. You can right-click the divider line and rotate the views to be either side-by-side or top-and-bottom.

As mentioned in the comments, there's a very handy compare plugin for Notepad++, that allows side-by-side comparison, indicating discrepancies.

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As well as moving or cloning the document to a separate view pane, you might want them to scroll together too, in a truly "side by side" manner. This makes it much easier to perform line-by-line comparison, for instance.

To achieve this, go to View -> Synchronize Vertical Scrolling (or Synchronize Horizontal Scrolling).

This may be sufficient for many, without requiring the need for the Compare plugin others have mentioned.