title: Merge two data frames with taking max of two columns
body: <p>I have two dataframes with the same form:</p>
  
  <pre><code>&gt; df1
  Day ItemId Quantity
    1      1        2
    1      2        3
    1      4        5
  
  &gt; df2
  Day ItemId Quantity
    1      1        0
    1      2        0
    1      3        0
    1      4        0
  </code></pre>
  
  <p>I'd like to merge df1 and df2 and if a row of <code>['Day','ItemId']</code> exists in both df1 and df2 take df1 which the max
  I tried this command : </p>
  
  <pre><code>df = pd.concat([df1, df2]).groupby(level=0).max(df1['Quantity'],df2['Quantity'])
  </code></pre>