On a failed save, a creature takes 10d4 acid damage and 5d4 acid damage at the end of its next turn. On a successful save, a creature takes half the initial damage and no damage at the end of its next turn.
If a 7th level Wizard/ 2nd level Fighter uses his Action Surge to cast this spell twice on the same target, and it fails both saving throws, how much damage does it take at the end of its next turn?
What if two casters use Ready to cast the spell at exactly the same time? In this case both the initial and the secondary damage comes at the instant.