Unique values of two columns for pandas dataframe

Suppose I have pandas data frame with 2 columns:

df: Col1  Col2
      1     1
      1     2
      1     2
      1     2
      3     4
      3     4

Then I want to keep only the unique couple values (col1, col2) of these two columns and give their frequncy:

df2: Col1  Col2  Freq
      1     1     1
      1     2     3
      3     4     2

I think to use df['Col1', 'Col2'].value_counts() but it works only for one column. Does it exist a function to deal with many columns?