text: Title: How to use pandarallel_apply with multiple arguments? 
  Body: I have code like below: 
  
  ```
  def similarity(txt1, txt2): 
      return xxxxxxx 
  vSimilarity = np.vectorize(similarity) 
  vSimilarity(df['var1'], df['var2']) 
  ```
  
  How to convert it to utilize parallel_apply? Below does not work.  
  
  `(df['var1'], df['var2']).parallel_apply(similarity)`