I don't know if this is philosophical or physics...but hopefully we don't have to go there:
The effects of the same spell cast multiple times don't combine, however.
So the question I have, is how do you clarify this with the fact that two wizards can cast a fireball (instantaneous) as a readied action at the same time? Do the rules clarify no two "instantaneous" things happen at the same instant, ever, in the whole world?
The only thing I can come up with, is that nothing in the game is supposed to happen at the same time.
This is not indicated for readied actions. So should you also resolve multiple readied actions with initiative rolls and d20 tie breakers?
Note: It seems this would also determine, for instance, a monster affected by an enchantment such that damage breaks the enchantment. I had ruled before that players can surround such a monster, each ready an attack, and each make one attack with advantage before the monster gets a chance. This approach adds realism and removes boring cleanup from the end of combats. However, if the readied actions can't happen at the same time, then it really is the first hit that would wake the monster up, and I am technically breaking rules with this.