User permission: document not visible if mulitple link fields of same type are present


I have a doctype Message that has two link fields (sender, receiver) to the same doctype Partner.

I want a user to see a document if either sender or receiver points to a partner that is assigned to the user by the user permission.

However, the document is only shown to the user, if ONLY the assigned partner is linked in the document.

That means that if I link in a sender and another receiver, the user will not see any document, because two different links are in the document. If I empty one of the link fields, the user can see the document.

Is there a way to use user permissions even if two link fields of the same type are present in a doctype?