What data source are you using? Are both tables in the same data source or different?
MS SQL Server data source. It was one data source with two different connections, as TAble A and TAble B are on the same server, different database. It was pointed out to me, that when you create the two connections, it is actually like two different data sources. So they suggested create one data source with one connection to Table A database. Link to Table B via custom sql and add the database to the code. This worked. When you try to link to two different databases in one data source, I always get the message that says Tableau requires a separate connection for the second database so I always did it that way.