title: Concat two Pandas dataframes without sorting on initial columns
body: <p>I have two data frames:</p>
  
  <p>Dataframe A</p>
  
  <pre><code>index  ID_1  ID_2   Value
  0       1     1      1.0
  1       1     2      1.2
  2       1     3      1.3
  </code></pre>
  
  <p>DataFrame B</p>
  
  <pre><code>index  ID_1  ID_2   Value
  0       1     2      1.0
  1       1     1      1.2
  2       1     3      1.3
  </code></pre>
  
  <p>And each is sorted by ID_1, then by Value.</p>
  
  <p>I want to merge or concatenate the two dataframes so that I can easily determine where ID_2 in dataframe A and dataframe B are not the same - so determine when the sorting I have highlights a difference in the order of the ID_2 column.</p>
  
  <p>I have tried to use pd.concat several different ways and I always get this result:</p>
  
  <pre><code>index  ID_1  ID_2   Value    ID_1  ID_2   Value
  0       1     1      1.0      1     1      1.2
  1       1     2      1.2      1     2      1.0
  2       1     3      1.3      1     3      1.3  
  </code></pre>
  
  <p>Rather than what I want:</p>
  
  <pre><code>index  ID_1  ID_2   Value    ID_1  ID_2   Value
  0       1     1      1.0      1     2      1.0
  1       1     2      1.2      1     1      1.2
  2       1     3      1.3      1     3      1.3  
  </code></pre>
  
  <p>Any ideas?  I have tried it with different column names, etc.</p>
  
  <p>Edit:</p>
  
  <p>The solution proposed below of course works, but one must remember to do <code>reset_index(drop=True)</code> on the dataframe to have the indexes reset after the sorting.</p>