Some of the things to consider are gender, size, and age. Most experts agree that, as a rule, male and female dogs get along better than two females or two males. However, that being said, I've known many households (including my own) that successfully have two of the same gender dogs without any problems at all.
For the happiest dogs and the safest household, opposite sex dogs almost always do best together. Many same-sex combinations of dogs will fight, sometimes to the death. Those who work out a dominance order may not fare much better.
Neutered male dogs might also fight. Or spayed females. Or unspayed females. If there is more than one dog, there is the potential for dogs to fight.
Allocate the Time and Resources for Another Pup
You also have to keep in mind that you'll need to invest extra for essential gear — collars, leashes and crates, to name a just a few items — as well as unexpected vet visits, potential boarding and possible pet sitters and dog walkers.
Gender: While there is no set rule that a female dog will be a better friend for a male dog, if you have never had two dogs at the same time it is generally easier and often safer to have dogs of the opposite sex. Same-sex combinations can be tricky to manage as dogs work out their dominance or pack order.
Let your dogs sniff each other and greet each other normally. Give them positive reinforcement through calm verbal affirmations. After letting them play for a while, put both dogs in a “sit” or “stay”, then let them interact again. Finally, take them on walks together, allowing them to sniff each other along the way.
Start on neutral territory where you have plenty of space, like a park, open field, or quiet street. Start on opposite sides of your space walking the same direction. When one dog looks at the other one, they get a treat. Keep walking until they are no longer focused on each other.
It can take up to one month for an old dog and new dog to really settle in and accept each other's position in the pack. If you want a second dog, you need to be ready to commit to this process and not panic.
Many same sex puppies get along throughout their lives. Many of mixed sex pairs do the same. I have found in my practice if fights/conflicts arise then two females can be the worst. Two males are less likely in fights/conflicts especially if they are both neutered.
It is a very poor decision to get two puppies from the same litter in almost all cases. Sometimes, people with extensive prior K9 knowledge may be able to handle the added stresses of getting two dogs from the same litter, but it is still generally not recommended.
In general, it's not recommended that two intact male dogs live together. This is because they are more likely to compete with each other for dominance, which can lead to fighting and aggression.
Dogs are social animals and usually happier around other dogs, but a second dog will never be a substitute for inattentive, absent or too busy owners.
In some cases, two dogs of the same sex may get along fine, but some experts feel it is best to choose a new pup of the opposite sex. Same sex dogs may become rivals, and there is a greater chance of competition among the two. If you want to play it safe, team your female dog with a male counterpart and vice versa.
Many breeders recommend that your first dog be at least one-to-two-years old before you add a second to the family. If you have an old dog, he may not be physically able to play with or tolerate a pup.
Dogs are inherently social animals that live well together in groups, but that does not mean that all dogs get along. Most dogs will welcome a new sibling, but it is not always smooth sailing. The furry family member you have now will face many changes when a new dog enters the picture and may feel a bit displaced.
Many people are hesitant to adopt a second dog because they assume it will be twice the work and twice the money. But the truth is, owning two dogs doesn't have to be more work – in fact, it can often be easier. And as for the cost, it will cost more money than one but not twice the amount.
In dogdom, there's a turn of phrase called, "Second Dog Syndrome". This describes the process of adding another dog to the home quite well, but not necessarily in a positive light. As humans, we are bound to forget all of the time and effort it takes to raise a puppy right.
Dominate behavior can occur in any breed of dog and it can occur in both males and females, though it's more common in males. In most cases, dominant behavior occurs when a dog has reached maturity, somewhere between one and three years old.
Under no circumstances should the dogs be allowed to "fight it out." You could be injured due to redirected aggressive attacks, or when you attempt to break up the fight (see below).
Many factors influence behavior, and not all siblings raised together will exhibit this problem, which is called “littermate syndrome.” In other words, it's a risk, not a foregone conclusion. That said, many dog behaviorists, trainers, breeders, and shelters actually discourage adopting siblings.
Taking on two puppies from the same litter often results in what is known as littermate syndrome. The puppies will be bonded to each other and this will make them less likely to bond to you. The dogs may not listen to you and will be much harder to train.
Research suggests that dogs are able to recognize their siblings and their parents later in life as long as they spent the first 16 weeks together. Intuitively, the less time dogs spend with their families as puppies, the less likely it is they'll be able to recognize a family member later on.