You have two cycles of 30 days, both perfectly out of sync with each other. However, one of the moon's cycles is actually 30 days and 2 seconds, so each year the other moon catches up to the first by just a few seconds. 100 years later, boom! they overlap. Now, you might be asking, if the distance only changes by a tiny bit each year, how can this double leap tide occurance be such a sudden and short-term event? Good question.
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