title: Merge and concatenate 2 dataframes at same time
body: <p>I'm trying to concatenate 2 dataframes keeping only those rows where value of 2 columns is present in both dataframes. For e.g.</p>
  <pre><code>tp1 = pd.DataFrame(
      {
       'A': [1, 2, 3, 4],
       'B': [5, 4, 2, 7],
       'C': [2, 4, 9, 1],
       'D': [1, 9, 7, 0]
       })
  
  tp2 = pd.DataFrame(
      {
       'A': [8, 2, 3, 9],
       'B': [6, 4, 2, 4],
       'C': [2, 9, 9, 1],
       'D': [1, 9, 7, 0]
       })
  
  tpOUT = pd.DataFrame(
      {
       'A': [2, 2, 3],
       'B': [4, 4, 2],
       'C': [4, 9, 9],
       'D': [9, 9, 7]
       })
  </code></pre>
  <p>if tp1 and tp2 are 2 dataframes then tpOUT is corresponding output.  I googled and found you can do this with pd.merge and pd.concat but cant seem to get it worked.</p>