title: python/pandas: how to combine two dataframes into one with hierarchical column index?
body: <p>I have two dataframes which look like this:</p>
  
  <pre><code>&gt;&gt;&gt; df1
                A    B
  2000-01-01  1.4  1.4
  2000-01-02  1.7 -1.9
  2000-01-03 -0.2 -0.8
  
  &gt;&gt;&gt; df2
                A    B
  2000-01-01  0.6 -0.3
  2000-01-02 -0.4  0.6
  2000-01-03  1.1 -1.0
  </code></pre>
  
  <p>How can I make one dataframe out of this two with hierarchical column index like below?</p>
  
  <pre><code>            df1       df2
                A    B    A    B
  2000-01-01  1.4  1.4  0.6 -0.3
  2000-01-02  1.7 -1.9 -0.4  0.6
  2000-01-03 -0.2 -0.8  1.1 -1.0
  </code></pre>