so, you are really trying to join 2 tables via a an intermediate formula...
Obviously, you can't join them directly. Can you create a view on table xxx and then join to the view to yyy? Can you get the data from a stored proc, that would also fix this?
OR
Create a Command Object and put the SQL statement that will return all the columns that you need and the join that works.
My order of preference is the 1) stored proc (get all the data you need and be done with it) 2) the view 3) the command object.
HTH