Krark's Thumb

• WWizards of the Coast2004-10-04
  Official Ruling
  If you and your opponent both flip at the same time, you can see your opponent's result before choosing which result to keep.
• WWizards of the Coast2019-01-25
  Official Ruling
  If an effect tells you to flip more than one coin at once, this replaces each individual coin flip. For example, if an effect tells you to flip two coins, you don't flip four coins and ignore any two; you flip two coins, flip two coins, and then ignore one flip from each pair of flips. You will know the results of all simultaneous flips before choosing which to ignore.

Rulings via Scryfall