it does work now
I noticed that you made them both primary keys. I don't think that'll work. Because your PK's are in the other 2 tables.
If you remove both primary keys, click on the relation view icon just above the index area and use the "link to" and link them to your PK's in the other tables you'll have just index keys on your 2 table items.
I think you need to make the tables innoDB as well, but not sure about that.
Thank you for your reply. You made good points.
But my idea was:
It is necessary to have them both defined (together) as primary keys because only each pair must be unique. I wonder if this option will be implemented in DA one day? I learned this trick from osCommerce which is an open source webshop soulution with some real good options.