Krark's Thumb — ruling 1

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.

Krark's Thumb — ruling 2

If you and your opponent both flip at the same time, you can see your opponent's result before choosing which result to keep.

Krark's Thumb : règles, fonctionnement et interactions (MTG)

Krark's Thumb is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

If you would flip a coin, instead flip two coins and ignore one.

How it works : Krark's Thumb

Original English text — Wizards of the Coast / Scryfall

#1January 25, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If you and your opponent both flip at the same time, you can see your opponent's result before choosing which result to keep.
No other card is named in this ruling.