Is a roommate allowed to live in the premise? Is the roommate on the lease? Is another dog allowed in the premises, what about roommate paying a pet deposit as well?
These are all landlord decisions, not yours, regardless of your personal reasons. If the landlord chooses, you may both be evicted without resolving the issues in writing with the landlord.
Doesn't matter who was on the lease first or second. Your fault for failing to train your dog of the aggressive behavior, training/correction are your responsibility, not the dogs, as is keeping everyone/everything safe from harm by whatever means necessary, should the dog injure anyone or anything, however slightly, it will be removed at your expense, you will be sued and the dog maybe euthanized. Hopefully you have renters insurance should this happen.