1. Please open 'System Preferences' and select the 'Mission Control' preference pane, where you will see an option labeled 'Displays have separate Spaces'
2. When this checkbox is selected, OS X will snap a window to whichever display that window has the most area on
3. When unchecked, OS X will allow a single window to span across multiple displays (including Duet) and will act as one large continuous display
- Any changes to this setting does require a logout -