title: Merging two dataframes with unequal numbers of rows by two columns names
body: <p>I am trying to merge two dataframes, lets say A and B, of unequal number of rows and columns. They have two common columns, c1 and c2. I know for a fact that every c1 x c2 combination that exists in one dataframe also exists in the other, so I'm not expecting any null values. A has many instances of each c1 x c2 combination that exists, whereas B has exactly one instance of each c1 x c2 combination. rbind and merge haven't been working.</p>
  
  <p>If the two data frames are:</p>
  
  <pre><code>  c1 c2 c3 c4
  1  A  1  5  1
  2  B  2  4  2
  3  B  1  3  4
  4  A  2  3  4
  5  A  1  3  3
  6  B  2  1  8
  </code></pre>
  
  <p>and</p>
  
  <pre><code>  c1 c2 c5
  1  A  1  5
  2  B  2  4
  3  B  1  3
  4  A  2  8
  </code></pre>
  
  <p>then I want to somehow bind with c1 and c2 and produce:</p>
  
  <pre><code>  c1 c2 c3 c4 c5
  1  A  1  5  1  5
  2  B  2  4  2  4
  3  B  1  3  4  3
  4  A  2  3  4  8
  5  A  1  3  3  5
  6  B  2  1  8  4
  </code></pre>