Dogs often find it difficult to live together if they are the same sex, entire, very different in personality or very similar.  If they both enjoy the same things, they may compete with each other over resources, especially if those resources are in short supply (or have been in the past) and neither will back down.

Problems usually occur during times of change.  For example, if a new dog is introduced to the household, when a puppy grows up and becomes an adult, during a house move, after a spell in kennels, or when a member of the family leaves the household or someone new arrives.

Within a family, a stable social structure exists where everyone knows their place and their role in the society.  Changes upset stable structures and force all members of the society to re-evaluate their place within the group.  This can sometimes lead to disputes and fighting as each member tries to hold on to the resources they need and want.

Dogs in multi-dog households often find themselves competing for attention from their owner, or a particular owner whose company they enjoy.  If your dogs begin to get cross with each other only if you are in present, and are perfectly okay when left alone or with other people, they could be fighting over the right to be near to you.

Dogs may also fight over resources, particularly if they are scarce or have been in the past.  Resources may include food, chews, bones, toys, and sleeping places.  Most dogs will eventually learn that they either can have everything they want since they are stronger and more determined than the others, or they learn that the others are stronger or more determined than they are and they can only have things when the other dog(s) does not want them.  Managing the ‘pack’ until they have reached this determination can be difficult.

Get professional help immediately to prevent injury.

Be extra careful if there:

• is a large differential in size as small dogs are easily killed (accidentally) by large dogs 
• are vulnerable people in the household who will be easily injured if the dogs fight, e.g. children or elderly

Coping with problems within a multi-dog household can be tricky and it really helps to ask a professional pet behaviour counsellor to observe and give you an impartial assessment and advice.  Ask your veterinary surgeon to refer you to someone he/she recommends or go to www.apbc.org.uk    

Although a lot of multi-dog problems can be solved with the right advice, some dogs just don’t get on and it may be kinder to consider rehoming.  This particularly applies to dogs that were raised alone with owners and have never had to share who may resent the other dog and never be able to accept them being nearby.  It also applies to littermates where one bullies the other, i.e. where one dog is very lively and outgoing and the other is very subdued  and gets pounced on by the other if it tries to do anything.  And it may also apply to dogs of the same sex who don't get on and have a long-standing animosity towards each other, especially two females living in the same household. 

Although it may be possible to have dogs living in separate parts of house while the situation calms and is worked on, it is not a good option long term and can be very stressful for all concerned.