Did you try to check why Tableau joined the second file with first one. Tableau will create joins on the basis of similar column name.
What you have to do is click on the joins and from drop list choose the right tables and columns. Once you choose them, Tableau will left join the table as you like.
Thanks! I just changed the field names and it worked. Sometimes you don't see the most "obvious" answer. It was all the time there.
Thanks for your help!