I do not know the answer to your question, but I would suggest opening a case with email@example.com if you haven't already. If you found a solution, be sure to post it here for others to find.
Also, just to clarify some terminology. We cannot "join" two different data sources in Tableau. Instead, we can use "data blending" to blend in data from two separate data sources. For more information on data blending, I'd suggest starting here: