Maximize the Left Viewport (Alt+W). Select the object then hold the Shift key and move it to the right. A new object is created!
In the Clone Options dialog choose Instance and rename it BoxInstance.
An Instance will have mirrored properties of the Master object and the Master object will have the same properties as the Instance. For example change the Stretch amount to 3.5 on the Instance and notice how the Master is changing shape as well.
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