You're missing the other dimension to this: it's not that two photographers took pictures at the exact same time, I'm sure that happens frequently. It's that they took the exact same picture. There were definitely not 10 trillion photos take of this lighthouse in new england.
They don't have to be photos of this lighthouse. Two photos of Old faithful showing the same spray and cloud patterns could be mistaken for copy's of each other.
Another way of thinking about it, what are the odds that out of the ~1,000 photos taken of the same moment they are of the same subject? Now repeat that question 10 million times and the odds don't seem as low.
