Most Dobermans play rough anyway. That and the fact that they are both males will pose a problem. Sometimes you will be lucky enough that two males will get along, but if you search the posts in this forum you will see majority of people saying 2 males just don't work in the same house, even littermates. Best of luck but keep them apart until your situation changes. I'm afraid the little beagle is not a match for a powerful Doberman. Even though you think he may be gentle having another male invade his territory might set him off. Don't give him the chance to do it.