title: Pandas - Merging on multi-index both ways
body: <p>I have two dataframes: <code>df1</code> and <code>df2</code>. Both have two indices, <code>A</code> and <code>B</code>. I can easily join them on their indices by doing <code>pd.merge(df1, df2, left_index = True, right_index = True, how = 'left')</code>. However, I want to not only merge <code>df1</code>'s <code>A, B</code> on <code>df2</code>'s <code>A, B</code>, but I also want to merge <code>df1</code>'s <code>B, A</code> on <code>df2</code>'s <code>A, B</code> (flipping the index order for <code>df1</code>). </p>
  
  <p>Is there a simple way to do this? I want to merge on the indices for performance reasons.</p>