But you are right that optimizing specifically for two files seems wrong, especially as the data contains a timestamp, so you could simplify the problem and ignore the number of files completely.
> Now, given two N log files we want to generate a list of ‘loyal customers’ that meet the criteria of: (a) they came on ALL days, and (b) they visited at least L unique pages.
while keeping linear time complexity and
O(num records in first file * L)
That means that multiple files doesn't need out-of-core if the maps for one file at a time fit memory.