reborg@www:~$ cat file
one two three four five
one two three four five
one two three four five
one two three four five
one two three four five
one two three four five
reborg@www:~$ sed 's/^\([^ ][^ ]*\) \([^ ][^ ]*\)/\2 \1/' file
two one three four five
two one three four five
two one three four five
two one three four five
two one three four five
two one three four five
reborg@www:~$