I'm afraid statisticly it just doesn't work that way. A mortor or howitzer generally hits in a somewhat circular pattern. A crater does provide better protection than just lying at ground level, but the idea of "chasing splashes" as the Royal Navy used to do, works only if you know the bad guys are adjusting fire. It has nothing to do with the chance of two shells landing on the same spot.
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"There is nothing so frightening as ignorance in action."