if you feed the signal line AND its ground return line TOGETHER through the ferrite!!.

I disagree.
If the return is a "ground return" and you filter it, you get unpredicatble results since you then depend on the grounding of two units (usually to different points). I even recall a case where Steve? posted that he burned a board because of ground differences.
If the "ground return" provide the grounding of e.g. a sensor a filter in the wire can be disastrous.
Erik