Quoted from Law:It's per game, and definitely carries over. Getting the multi-kills relit after a good ball can be worth a lot more than multiball.
Multiball also adds to multikill value so don't ignore it! If you can manage an inlane X coupled with 2x super jack on prison or well (Using a bomb makes this much easier) your MK values will skyrocket and in turn the Horde value.