In order to join two CSV files you click on the three dots behind CSV file first. The following window appears:

 

 

Click on the three dots again.

 

 

Now you choose the desired CSV file and click on Open.

 

 

Click on Next.

 

 

Choose the column delimiter of your CSV file as well as the text qualifier and the encoding. Also make sure that you tick Column names in the first line when you want to take over the data of the first row.

 

Then you click on Next.

 

 

Finally, you click on Finish. You can now see the file in the left part of the Join CSV files window:

 

 

For the second CSV file you do the same.

 

Once you have uploaded both files you can continue with the Field settings.