After Acolyte's Reward resolves, it no longer matters whether either target is still legal. For example, if the second target is a creature controlled by an opponent, and it gains hexproof after Acolyte's Reward resolves but before it prevents damage, Acolyte's Reward will still deal damage to that creature. If Acolyte's Reward can't deal damage to the second target (perhaps because it's a creature that has left the battlefield), Acolyte's Reward will still prevent damage; it just won't deal any damage itself.