[eluser]WanWizard[/eluser]
You're trying to get something related on an empty object? Assuming you want the source records, you should have reversed your question.
I'm not sure (actually, I don't think so) that you can get both source objects in one go. When using relationships, you have to specify (in case multple exist), which relation you want to query.
Maybe you should explain what it is that you want to get out of this query?
