New dog introductions can be difficult and traumatic if one dog is aggressive towards the other. There are a few training tips that can help make this introductions easier. I know that they’ve already been ‘introduced’ but a ‘re-introduction’ may be necessary and is outlined below. Keep in mind that inter-dog interactions are complex and a trainer or behaviorist may need to be consulted in some cases.
Introductions should always take place in a ‘neutral’ space. Basically this means that the dogs are being introduced in a setting that doesn’t ‘belong’ to one dog or the other. This can be outdoors at a city park that is unfamiliar to the dogs or in the kitchen where both dogs are fed together. Some dogs will become territorial and aggressive if a new dog is suddenly introduced into their ‘space.’ After a few ‘introductions’ in a neutral space, you can continue this process in the home that you will share together.

Keep both dogs leashed during the entire introduction. This will ensure their (and your) safety. ALWAYS keep yourself safe as well. Many people are bitten or injured trying to separate fighting dogs. Be sure that their collars and leashes are fitted properly. If the dogs have had poor obedience training and are difficult to control on the leash, this process may be more difficult. 

Have both dogs sit and focus on YOU (the handler) during the first part of the introduction. When they are calm and focusing on you, reward with a small treat. I like to use Cherrios oat cereal as a training treat. They are small, easy to carry in your pocket and low-calorie. One or two is just enough to tell your dog that they’re being rewarded.

Have the more dominant dog (Sandy) lay down as Storm approaches. Again, be sure that you have a firm grip and control over them to prevent an attack. Roll Sandy onto her back and hold her there. If she is calm, reward her with treats. Slowly bring Storm over close to her and have them look each other in the face. Doing this exercise at least once a day can help change their relationship with each other and reinforces that Sandy needs to be calm and ‘submissive’ when Storm is around. Reward Storm too for being brave and approaching Sandy. Rewarding him can help reduce his fear response to Sandy over time.

As mentioned before, this is a good starting place for introducing dogs. Every case is different and it is difficult to make recommendations in writing without seeing the dogs interact. If you try this method for a couple of weeks and there is no improvement, please consult a local dog trainer or behaviorist.