Rank (col1, col2) issue?

[mc09]

hi
  i implement below logic in my report
1 create one query subject 'ranking'  with expression "Rank (col1, col2)" 
2.set pre-sorting as Sort ASC
  when i testing, it worked for col1 wile it is disorder in col2 when they have the same col1 value.
  how to implement sort by col1 firstly and then sort by col2?