Add a third field as a text field.
Give it an auto entered calculation such as: WorkShop# & " " & OrderID
Define a unique values validation rule for it.
For other methods to prevent duplicates in a join table as well as an example of this method, see this demo file: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
Thanks a lot, I'll do that.