title: Pandas Set multiple column and row values to nan based on another dataframe
body: <p>I have two large dataframes, <code>df1</code>:</p>
  
  <pre><code>Col1    Col2    Val1    Val2    Val3
  asd     ASYL    4.2             4.2
  ppq     CONE            35      35
  DA      HU      100     100
  </code></pre>
  
  <p>And <code>df2</code> is</p>
  
  <pre><code>Col1    Col2    Val1    Val2  Val3
  asd     ASYL    7       12    17
  ppq     CONE    17      19    19
  DA      HU      5       14    13
  </code></pre>
  
  <p>Both dataframes have same index columns <code>Col1</code> and <code>Col2</code> and same value columns <code>Val1, Val2, Val3</code>.</p>
  
  <p>I want to set the values in <code>df2</code> to <code>nan</code> where it is <code>Null</code> in <code>df1</code> to get the following:</p>
  
  <pre><code>Col1    Col2    Val1    Val2  Val3
  asd     ASYL    7             17
  ppq     CONE            19    19
  DA      HU      5       14      
  </code></pre>
  
  <p>I tried the following:</p>
  
  <pre><code>idx = df1.isnull()
  df2.loc[idx] = np.nan
  </code></pre>
  
  <p>But it is not working.</p>