you would use 2 Insert behaviors.
the insert behavior will store the ID of the record it creates in a session variable. In the second insert behavior, you would bind the relational column to the session variable created by the first insert.
Data Assist uses dreamweaver recordsets for getting information out of the database to display on the page, so will have the same limitations including BLOB Fields.
you can add as many dreamweaver recordsets to a page as you need, dreamweaver recordsets can also use MySQL Stored Procedures.