No other controller. Interestingly enough, the problem kinda “fixes” itself when you get the first str perk or discovering a new truncheon (hardened steel).
Before that, all blunt weapons deal wayyyyyyyyyyy more physical dmg than non lethal.
For me this happened also (official). no second controller. I had my thrall with me, who made the victim loose Health. I switched him fast to truncheon and use my own. We killed the victim with truncheons. Maybe this happens, when the victim has not full Health initially. Normally this is no Problem. But I experienced this after the last patch.