title: Dataframe question, concatenate 2 dataframes but avoid having empty rows
body: <p>I have a dataframe with 100 users and some small information. Now I want to create another dataframe and merge them, the new dataframe needs to have the same users but now includes new data and more data. so each user on the original dataframe now has 100 more datapoints in the new dataframe.</p>
  <p>example for 2 users, and 4 datapoints on the 2nd dataframe</p>
  <p>dataframe 1:</p>
  <pre><code>Name | Age | Username 
  John | 20  | johndoe01
  Johny| 24  | johnydoe02
  </code></pre>
  <p>Datafrane 2:</p>
  <pre><code>Username2 | Score | Accuracy
  johndoe01 | 80    |   70%
  johndoe01 | 50    |   50%
  johndoe01 | 80    |   70%
  johnydoe02 | 50    |   50%
  johnydoe02 | 20    |   30%
  johnydoe02 | 50    |   50%
  </code></pre>
  <p>Now , the second dataframe is easy to create , using the iterrow , however my goal is to  concatenate them so it looks like this :</p>
  <pre><code>  Name | Age |  Username2 | Score | Accuracy
    John | 20  |  johndoe01 | 80    |   70%
    John | 20  |  johndoe01 | 50    |   50%
    John | 20  |  johndoe01 | 80    |   70%
    Johny| 24  |  johnydoe02| 50    |   50%
    Johny| 24  |  johnydoe02| 20    |   30%
    Johny| 24  |  johnydoe02| 50    |   50%
  </code></pre>
  <p>again these are examples but I just want to basically create a 2nd dataframe using faker that generates multiple values per user from the first dataframe and then merge both dataframes  so they one big table.    My current approach is to use an iterrow on dataframe1 and per each row create a new dataframe with the same username and the other data, then concatenate this to the first dataframe and go on but does not give me the expected result which is the one above.</p>