Fight or Flight

Creatures which enter (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile. Since they weren't in the chosen pile, they won't be able to attack.

If you have two of these, then you will separate the creatures into piles twice, and the opponent will choose a pile each time. Only creatures that were in both of the chosen piles can attack.

The effect is best understood if you read it as "creatures not in the chosen pile can't attack."