title: How to merge overlapping columns
body: <p>I have two datasets like this</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df1 = pd.DataFrame({'id': [1, 2,3,4,5], 'first': [np.nan,np.nan,1,0,np.nan], 'second': [1,np.nan,np.nan,np.nan,0]})
  df2 = pd.DataFrame({'id': [1, 2,3,4,5, 6], 'first': [np.nan,1,np.nan,np.nan,0, 1], 'third': [1,0,np.nan,1,1, 0]})
  </code></pre>
  
  <p>And I want to get</p>
  
  <pre><code>result = pd.merge(df1, df2,  left_index=True, right_index=True,on='id', how= 'outer')
  result['first']= result[["first_x", "first_y"]].sum(axis=1)
  result.loc[(result['first_x'].isnull()) &amp; (result['first_y'].isnull()), 'first'] = np.nan
  result.drop(['first_x','first_y'] , 1)
  
    id    second  third   first
  0   1   1.0      1.0    NaN
  1   2   NaN      0.0    1.0
  2   3   NaN      NaN    1.0
  3   4   NaN      1.0    0.0
  4   5   0.0      1.0    0.0
  5   6   NaN      0.0    1.0
  </code></pre>
  
  <p>The problem is that the real dataset includes about 200 variables and my way is very long. How to make it easier? Thanks</p>