If he doesn't behave himself when the roomie is there, I would place him in a time out in his kennel. Dobies hate not being part of the action. If he wants to be with you he has to behave. You have to ask yourself how do I want the dog to behave in this situation and what command is he going to complete to show me he is behaving? Have him do his obedience commands. Put him in a down when the roomie comes in. Yes Dobies are stubborn always pushing the envalope of what they can get away with. and yes they overthink everything. Get used to it
