Splendid Agony

• WWizards of the Coast2017-04-18
  Official Ruling
  You choose how the counters will be distributed as you cast Splendid Agony. Each target creature must be assigned at least one counter.
• WWizards of the Coast2017-04-18
  Official Ruling
  If one of two target creatures becomes an illegal target in response to Splendid Agony, the -1/-1 counter that would have been put on that creature is lost. It can’t be put on the legal target.

Rulings via Scryfall