title: How do you merge two Pandas dataframes with different column index levels?
body: <p>I want to concatenate two dataframes with same indices but different column-levels. One dataframe has a hierarchical index, the other on doesnt.</p>
  
  <pre><code>print df1
  
                A_1               A_2               A_3                .....
                Value_V  Value_y  Value_V  Value_y  Value_V  Value_y
  
  instance200   50       0        6500     1        50       0
  instance201   100      0        6400     1        50       0
  </code></pre>
  
  <p>the other one:</p>
  
  <pre><code>print df2
  
                PV         Estimate
  
  instance200   2002313    1231233
  instance201   2134124    1124724
  </code></pre>
  
  <p>result should look like this:</p>
  
  <pre><code>             PV        Estimate   A_1               A_2               A_3                .....
                                    Value_V  Value_y  Value_V  Value_y  Value_V  Value_y
  
  instance200  2002313   1231233    50       0        6500     1        50       0
  instance201  2134124   1124724    100      0        6400     1        50       0
  </code></pre>
  
  <p>but a merge or concatenate on the frames will give me a df with a one-dimensional column index like that:</p>
  
  <pre><code>             PV        Estimate   (A_1,Value_V) (A_1,Value_y) (A_2,Value_V) (A_2,Value_y)  .....
  
  
  instance200  2002313   1231233    50             0             6500         1
  instance201  2134124   1124724    100            0             6400         1 
  </code></pre>
  
  <p>How can i keep the hierarchical index from df1?    </p>