This may be a small unexpected feature. Open an NWC file with the Viewer and then drag-and-drop further files on the viewer window to open them in turn. Pressing the toolbar button "Open in NWC" (the version 1 software) does not open the file currently being viewed but opens the file that was first opened. If you ONLY use drag-and-drop to open files then the toolbar button is not active and you cannot open the visible file in NWC.
This is not as I would expect but in the whole scheme of things is a small problem with what is otherwise a very good tool.
This was fixed about a week ago. Try installing the latest version (http://www.noteworthysoftware.com/nwc2/viewer.htm).
Thanks - new version downloaded. I'll keep an eye open for new versions!