Mirror Tool Options

Merge mirrored objects	When you use a face as the mirror plane, or when the mirror plane lies on a face of the source object, the two mirrored objects will be merged. Deselect this option to make a new object, as shown below.
Create mirror relationships	If you deselect this option, only the geometry is created and not the mirror relationship. Changes to either object will not be reflected in the other, as shown below.

You can create a mirror plane between two symmetrical faces in your design.