I can’t say that what you’re asking is impossible, but far as I know the only way to select/manipulate symmetrical attributes of an object is with the Symmetry object. Otherwise, I don’t know how C4D would be able to correlate one side of an object with the other and select and modify the appropriate polygons, points or edges. The Symmetry object is obviously just creating a mirror image of its child object, and manipulating both halves, but without doing that I don’t see how to accomplish what you’re looking for. If you find a way, please pass it on. Good luck.