What kind of hens do you have? Some of the bigger heavier breeds can have this problem. Are you sure it's a "crush" and not a matter of the bird(s) cracking the egg to eat it and then the remaining shell being crushed once the integrity of the shell is broken? I mean, if you are out there to see the first bird lay, leave and the second bird enter the nest and crush the egg I'd just prevent it by collecting the egg - which I assume you aren't out there at just the right moment each day and that is why I ask if you are sure it is being crushed vs. other things happening to it.
You can set the nest box up to have a slight tilt and a "safety box" where the eggs will roll to the safe area rather than remaining in the main area where the hens bodies are
