You need to start right now and establish that neither of them is alpha - YOU are! Keep a spray bottle of water handy, if that will distract them from fighting, use it. When they start squabbling, redirect their attention to you, and away from each other. The bulldog may have no cepncept that she is not as big as the Dobe - some smaller dogs never do understand that!

This is not an unsurmountable problem, but it will be work for the humans in the family.