I currently hold two duchies. If I give one of my sons a county in one of the duchies, then he becomes my vassal, right? And if that duchy is then inherited by my other son, then one of my sons will be vassal to the other, right? Except that both of my sons will be dukes – each inherits one duchy – and one duke can't be vassal to another!
Taking it further, what if I give each son a county inside the duchy that the other son will inherit? When I die, each should become the other's vassal!