to expand on what Ian is saying slightly:
In the DataAssist Insert Record behavior, there is a section titled Session variable. In the first insert record, insert the client data, select the ClientID as the Key Column in the Session Variable section and make note of the name in the Store As setting.
when the Client information is stored, it will store the ID of the record that is created.
in the second insert record behavior, it will insert to the cases table. Bind the casesClientID column to the session variable created by the first insert behavior.