Instructions
Choose another chihuahua as a second dog if your first dog is also a chihuahua. Although chihuahuas regard other dogs as rivals for their master's affection, they make an exception for their own breed. Two or more chihuahuas rarely have any problems co-existing in the same household, and often become great friends.
Bring a chihuahua in as the new dog; do not bring another breed in to your home if you already have a dog that is not a chihuahua. Multiple dogs in a family also are much more likely to get along if they are brought in at the same time, or if they have been in the same family from puppyhood.
Allow the new dog and the ones you already have to socialize outside the home, in a neutral place like a dog park, before you bring the new dog home. Do this more than once. The advantage of a dog park over other outdoor locations is that you can let the dogs go off-leash. Territorial dogs like chihuahuas will interpret the leash as an extension of their owner, and will be more inclined to show aggression toward other dogs who come near than if both are permitted to interact independently. Always take time to get a sense of how other dogs in the park behave with each other before taking your dog's leash off.
Bringing a Chihuahua Into a Home With Other Dogs
Chihuahuas are a common breed in large part because of the intense bond they form with their human caretakers. But the flip side of that loyalty is that these dogs tend to be jealous and territorial. This does not mean you cannot bring a chihuahua into your home if you already have a dog. However, there are some steps you should follow to increase your chances of success.