If the second target creature is no longer on the battlefield as the damage is dealt, the damage isn't redirected away from its original recipient. The same is true if the second target creature isn't a creature (or a planeswalker) at that time.

You choose the two target creatures as you cast Kor Dirge, but you choose the source as it resolves.

The damage dealt to the second target creature is dealt by the original source of damage, not by Kor Dirge.