The -t switch sorts the output based on [c s], column C using character S as the columns separator
--
/filter -fftu
1 1 file1.txt file2.txt * <- to sort 1,2,3,4,
/filter -fftue
1 1 file1.txt file2.txt * <- to sort 4,3,2,1
If there are no 'columns', make up some

;)
edit-
Added the forgotten "t" switches
