Thank you, it's an interesting idea. If I understand you correctly, you want to manually specify a multi-column unique constraint via the UI?
In the database it would be something like this:
CREATE UNIQUE NONCLUSTERED INDEX [NC_Relationship11_TwoUniqueConstraints1] ON [dbo].[Relationship11_TwoUniqueConstraints] ([one], [two])
I like the suggestion, I will add it to the list and see what we can do for a future release.