Something

#process the list to find unique pairs
while IFS=$TAB read rtr1 rtr2; do
  unset order
  while read rtr; do
    order=( ${order[@]-} $(echo $rtr) )
  done< <(echo -e "$rtr1\n$rtr2" | sort -u)
  echo -e "$rtr1\t$rtr2\t${order[0]}\t${order[1]}"
echo
done < ~/tmp/neighbors2 | sort -u