title: Merging 2 arrays to get unique values in python
body: <p>Got 2 df columns which I would like to merge together. Since most of the values are equal across corresponding rows I would like to create a single column with the unique value from each row. </p>
  
  <p>What's the best way to achieve the below output?</p>
  
  <p>Sample:</p>
  
  <pre><code>list1 = ['','','dog','cat','cat']
  list2 = ['dog','','dog','cat','']
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>combined_list = ['dog','','dog','cat','cat']
  </code></pre>