from the help:
If a stream area is selected, it is possible to add another, disjointarea by Ctrl-clicking its start point and moving the mouse to the endpoint. More than one extra selection can be added this way.(Multi-Editing should be enabled in Preferences)
Source: http://npp-community.tuxfamily.org/documentation/selecting-text
tl;dr - Settings -> Preferences -> Editing -> Enable Multi-Editing -> check. Then Ctrl-Click for each section.
