I think this commendation is hard enough to exploit and easy enough to balance even as it is now (except for double kills Delian mentioned, and since reactions should be also awarded for non-lethal, I second that only the person who neutralized the enemy should gain it, either by killing or knocking out).
Speaking of KO's, there is commendation logic that offers much bigger exploits: STATUS_UNCONSCIOUS 'kill' is awarded to all player units that participated, even if the enemy is KO'd multiple times per mission or even per turn (IMO should only go to the first time in battle). Of course few have the required level of self-loathing presented by some players from Russian comunity, creating stun-revive-stun trains...