title: Combine two Pandas dataframes with the same index
body: <p>I have two dataframes with the same index but different columns. How do I combine them into one with the same index but containing all the columns? </p>
  
  <p>I have:</p>
  
  <pre><code>  A 
  1 10 
  2 11
  
    B
  1 20
  2 21
  </code></pre>
  
  <p>and I need the following output:</p>
  
  <pre><code>  A  B
  1 10 20
  2 11 21
  </code></pre>