Southpaw,
If you use the Mirror option or flip a copy of an object with the handles, tweaking or moving the primary object will cause the mirrored effect to happen to the copy.

Symmetry as a d-sub modeler uses is really not feasible in MoI
unless 1) you were allowed to edit surface and curve points forever (there seems to be a point where an object doesn't let you see the points anymore), and also made larger concentration of points more manageable such as with a soft election or rubber band method.
..and 2) or MoI employs the use of Components or active clones where changing one changes all, including additions of objects within the component (even it those clones were "mirrored").