title: Pandas: combining header rows of a multiIndex DataFrame
body: <p>Lets say I have a DataFrame as follows:</p>
  
  <pre><code>first   bar       baz       foo
  second  one  two  one  two  one  two three
  A       1    2    3    4    5    6   7
  B       8    9    10   11   12   13  14
  </code></pre>
  
  <p>and I want to create a new DataFrame like this:</p>
  
  <pre><code>        barone  bartwo  bazone baztwo  fooone footwo foothree
  A       1       2       3      4       5      6      7
  B       8       9       10     11      12     13     14
  </code></pre>
  
  <p>What would be the possible code?</p>