You should always introduce your dogs in stages. Start by introducing them on either side of a closed door, then through a baby gate, and then on a tandem walk. If either dog growls or barks, you should take a step back in the process and continue to use treats/rewards to help the dogs develop a good association. Furthermore, each dog’s body language will help you determine how they’re feeling. Areas to pay attention to include each dog’s:
Tail. A loose tail or propeller-like wag usually indicates your dog is feeling friendly. On the other hand, a stiff or high tail may indicate that your dog is on guard.
Ears. Ears held in a natural position typically indicate that your dog is relaxed. On the other hand, ears that are held up and forward may indicate a willingness to fight or general aggression towards the other dog.
Eyes. “Soft Eyes,” a warm, sometimes squinty look, usually means your dog is friendly and willing to interact. On the other hand, “Hard Eyes,” a cold, icy look usually associated with a direct stare, usually indicates that your dog may be prepared to fight or aggress forward.
Mouth. A mouth that is slightly open, with no teeth exposed, generally means your dog is relaxed. Whereas a frightened dog may pull the corners of his lip back (“lip long”) and a dog that needs space may pull their lips back, bare their teeth and wrinkle their muzzle. This action is designed to intimidate the other dog from moving closer.