You don't even need to resort the files if they are already sorted, as sort can merge sorted files:

From (textutils.info)sort:

`sort' has three modes of operation: sort (the default), merge, and check for sortedness. The following options change the operation mode:

`-m'

Merge the given files by sorting them as a group. Each input file must always be individually sorted. It always works to sort instead of merge; merging is provided because it is faster, in the case where it works.

This means you won't have all six files open at once: you open two for reading, merge them into a temporary file, merge the third with the result etc.

Comment on Re^2: join on 6 huge files