First, a person will register the face with the phone, confirming that he can unlock it by looking at it, then pass the phone to the other person.
For both twins, the other one can unlock the iPhone X. The Franklin brothers both removed glasses and Face ID were indistinguishable.
Apple has never said that Face ID is perfect, but at least it has not been fooled by images or videos. But for twins, the results will make them think. By the way, Windows 10's Windows Hello recognition technology is also going through this challenge but the other is that it succeeds in distinguishing two twins.
Because Face ID uses chips and powerful algorithms - can learn how to change the face over time - it is hoped that its ability to identify twins will also improve.