title: pandas: merge (join) two data frames on multiple columns
body: <p>I am trying to join two pandas data frames using two columns:</p>
  
  <pre><code>new_df = pd.merge(A_df, B_df,  how='left', left_on='[A_c1,c2]', right_on = '[B_c1,c2]')
  </code></pre>
  
  <p>but got the following error:</p>
  
  <pre><code>pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4164)()
  
  pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4028)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13166)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13120)()
  
  KeyError: '[B_1, c2]'
  </code></pre>
  
  <p>Any idea what should be the right way to do this? Thanks!</p>