I want to adopt a second dog (we already have a very dominant female). As BenMax mentioned, it's hard to comment without seeing what you're dealing with, but here's what I've found...
I'm interested/concerned about this, so I've been reading about it:
- Off territory outings are best and should be done immediately after acquiring the second dog, and often after that
- separate areas for feeding and removal of feeding bowls
- one-on-one training with each dog separately
- *this last one I find the most controversial*: Agreeing with your first dog when he/she puts the new dog in it's place...
I'm guessing that the idea here is that dogs are used to hierarchies. Your dog already 'owns' everything in your house that isn't the Alpha's (yours/your family's). They also understand that the current hierarchy is: Owner/family/people #1, Me(dog) #2. Intro-ing a new dog puts the other on edge as they don't understand where this new dog fits into the hierarchy. Are they here to steal? To cause harm? To boot your dog off the #2 position?
By being aggressive to the new dog, yours is communicating the existing hierarchy (maybe over-communicating for effect). When you scold her, it's confusing: she's confused that you don't appear to follow the same hierarchy as she thought. She's unsure now where she stands in the ranks (and so may have to fight more to secure her #2 spot). Scolding her, and feeling bad for/comforting the new dog also gives your dog insecurities on her position, and invites the new dog to continue his behavior (smelling her/taking her toys, etc).
Some dog trainers recommend that, if the fights are not severe, you should praise your current dog for putting the new one in it's place. Ex: New dog takes old dog's toy. Old dog saps at new dog, takes back toy. Praise your dog and ignore new dog. (New dog prob also appreciates this in the long run, as they also understand how they fit into the family).
I'm not sure that this idea would work in all practical situations. But I think the idea of hierarchies is a good one to keep in mind. Also keeping in mind exactly what they are fighting about - toy? - your affection? Your current dog will prob need more affection to let them know they are still #2 in the ranks.
If it's any consolation: My family had a male mini-schnauzer who was a couple years old when we got a golden retriever (male) puppy. The schnauzer picked on him for about the first year - really fighting him, biting at his face. Then the golden got a lot bigger (maybe a shift in hierarchy?). After that, they were the closest buddies - went everywhere together.
Best of luck! Please keep us updated on their progress.