If you are joining two OBIEE reports by creating a join relationship in the same data connection/source, you will see an error in Tableau.  This is because Tableau would send a query to OBIEE as if they are both from the same schema or report which is incorrect.  

The correct approach is to use Tableau's database join capability. You must create two data connections - one for each report and then create a join as needed.  Then Tableau would send individual queries to each of the data sources (in this case - OBIEE report), fetch the data, and then join them in-memory.  Attached is a short video of how this done.