Antagonist will have a 50% chance to deflect a bullet and grant damage reduction from the bullet deflected. If it succeeds in deflecting then KR's deflection damage is ignored and Antagonist's deflection damage will be used. However, if it fails to deflect a bullet then KR will have a (100%) chance of deflecting the bullet. If it does (and it will) deflect the bullet then KR's deflection damage will be used, ignoring Antagonist's deflection damage.
In short, the damage from the deflected bullet will not stack.
Aye you guys, aye! Y'all got your helmets on backwards... Jus sayin.