Synchronize window views
Window synchronization is turned on by default. Choose WindowDisplay > Window Sync 
to turn it off.
Choose Window Sync again to turn synchronization back on.
Note:
This normally applies to the orthographic windows only (Top, Side, Back, etc.). However, it works in the Perspective window if Perspective is turned off in the Viewing Panel or ViewCube and one of the preset orthographic views (horizontal or vertical arrows) is selected.