Hi,
In the background of WP Related Items plugin YARPP ensures the similarity finding functionality. In this environment only bidirectional relationships can be handled, so if two items are related, then it is true in both direction.
Manual relationship handling is part of this functionality, thus manual relationship is also bidirectional.
Due to the above, unfortunately it is not possible what you would like.
If you have more questions, please do not hesitate to contact us.
Regards,
Peter Rath
WebshopLogic