title: Convert values in one dataframe to NA based on NA's in a second dataframe
body: <p>I'm sure there is a very easy way to accomplish this task, but I cannot seem to figure it out. I have two data frames which have the exact same data but from two separate locations. </p>
  
  <pre><code>df1 &lt;- data.frame(a=c(1,2,3,NA),b=c(1,5,4,6))
  df2 &lt;- data.frame(a=c(3,4,5,6),b=c(7,8,9,NA))
  </code></pre>
  
  <p>My desired output is two have a new version of df1 and df2 which are the exact same but the bottom row only contains NA values. I.e. if there is an NA value in one data frame, I need that replicated on the corresponding cell in the other data frame...</p>
  
  <pre><code>df1[4,2] &lt;- NA
  df2[4,1] &lt;- NA
  </code></pre>
  
  <p>I have seen very similar questions addressing the problem from the opposite perspective (e.g. <a href="https://stackoverflow.com/questions/37015659/filling-missing-values-in-a-data-frame-from-another-data-frame">Filling missing values in a data.frame from another data.frame</a>) but I can't figure how to apply this to my own data. Thank you in advance. </p>