As Acolyte's Reward tries to resolve, if only the first target is illegal, Acolyte's Reward won't prevent any damage that would be dealt to that creature and, because of this, Acolyte's Reward won't deal damage to the second target. If only the second target is illegal, damage that would be dealt to the first target will be prevented, but Acolyte's Reward won't deal damage. If both targets are illegal, Acolyte's Reward won't resolve.