I would have to think that all enemies hit spawn a ‘second’ shot. If not, then it’s only the first enemy hit.
Edit: Upon testing in the Dojo, it does 50 damage in a really strange radius that doesn’t scale with Attack damage or Skill Damage. (Wasn’t hitting minions right next to the target, but was damaging minions further away.)
Double Dose starts at 50 damage for the second shot, but does have scaling damage with level/Attack damage. It seems to only ever spawn one dart per shot fired, I believe. (Hard to test on the Dojo minions though.) It also applies the wound, I believe, though not the DoT.