The first job of caregivers deciding to join two canine families is to have all dogs assessed for health problems. Any disease or pain-related condition might render dogs more irritable and hence more likely to engage in aggressive behaviours. Pain needs to be treated.
Dogs who already have behavioural concerns may have these problems exacerbated by the stress of combining households.
Remember you are asking unrelated individuals, with no genetic link or previous connection, to suddenly spend large amounts of time together, whilst sharing space and attention. This may be stressful (even for normal dogs). And people.
If time is on your side it is best to prepare well ahead and teach all parties to respond to cues such as “come away”, “look” and “go” to a safe place. The more well-learnt and routinely practiced these behaviours the more likely you will be to be able to use them if tension rises. Gradual introductions work best. First introductions should be off property. Avoid cramped spaces. Avoid competition around highly valued resources e.g. food, attention, games.
There is no need to decide which dog is dominant. Dogs live in "family groups" that do not mimic a wolf pack.
Dogs have fluid hierarchies and behaviourally normal dogs do not need to control the movement, or the access to resources of other dogs, all the time.
Normal dogs do not fight to the point of injury to maintain access to resources. Normal dogs may have an occasional vocal squabble, but injury should not be part of that scenario and recovery is quick.
Dogs who have behavioural concerns such as anxiety, dog-to-dog aggression or resource-guarding tendencies will be at more risk of finding sharing space problematic. In preparation for living together dogs can be desensitised to muzzle wearing (Baskerville type) and have well trained, happy crate behaviours in place.
The essence of blending a family of dogs is to teach all dogs that good things happen in the presence of other dogs.
Dogs must be calm enough to both look to caregivers for instruction and to be able to take food for being calm. This work may start with dogs on lead wearing head halters to direct attention. Dogs must be able to be within a few feet of one another and calm enough to listen to caregivers’ cues. Caregivers must remain calm.
For some dogs medication may assist with learning calm behaviours and improving impulse control.
Pheromone diffusers and collars may also assist some dogs make the transition.
Dogs should not be left together, unsupervised, to “work it out.”
No one should reach between fighting dogs. Serious injury can occur. In households where young children reside it may not be appropriate to mix some dogs. Sometimes rehoming a dog may be the best option.