title: Pandas concatenate alternating columns
body: <p>I have two dataframes as follows:</p>
  
  <pre><code>df2 = pd.DataFrame(np.random.randn(5,2),columns=['A','C'])
  df3 = pd.DataFrame(np.random.randn(5,2),columns=['B','D'])
  </code></pre>
  
  <p>I wish to get the columns in an alternating fashion such that I get the result below:</p>
  
  <pre><code>df4 = pd.DataFrame()
  for i in range(len(df2.columns)):
      df4[df2.columns[i]]=df2[df2.columns[i]]
      df4[df3.columns[i]]=df3[df3.columns[i]]
  
  df4 
  
      A   B   C   D
  0   1.056889    0.494769    0.588765    0.846133
  1   1.536102    2.015574    -1.279769   -0.378024
  2   -0.097357   -0.886320   0.713624    -1.055808
  3   -0.269585   -0.512070   0.755534    0.855884
  4   -2.691672   -0.597245   1.023647    0.278428
  </code></pre>
  
  <p>I think I'm being really inefficient with this solution. What is the more pythonic/ pandic way of doing this?</p>
  
  <p>p.s. In my specific case the column names are not A,B,C,D and aren't alphabetically arranged. Just so know which two dataframes I want to combine.</p>