title: Pandas - merge two DataFrames with Identical Column Names
body: <p>I have two Data Frames with identical column names and identical IDs in the first column. With the exception of the ID column, every cell that contains a value in one DataFrame contains NaN in the other. 
  Here's an example of what they look like:</p>
  
  <pre><code>ID    Cat1    Cat2    Cat3
  1     NaN     75      NaN
  2     61      NaN     84
  3     NaN     NaN     NaN
  
  
  ID    Cat1    Cat2    Cat3
  1     54      NaN     44
  2     NaN     38     NaN
  3     49      50      53
  </code></pre>
  
  <p>I want to merge them into one DataFrame while keeping the same Column Names. So the result would look like this:</p>
  
  <pre><code>ID    Cat1    Cat2    Cat3
  1     54      75      44
  2     61      38      84
  3     49      50      53
  </code></pre>
  
  <p>I tried:</p>
  
  <pre><code>df3 = pd.merge(df1, df2, on='ID', how='outer')
  </code></pre>
  
  <p>Which gave me a DataFrame containing twice as many columns. How can I merge the values from each DataFrame into one?</p>