You have one dog. He’s the best dog you’ve ever had. He sits alone in the house all day while you’re at work 
and you feel bad for him. Someone suggested bringing another dog into the house so he’ll have a buddy to keep him company. You don’t have time for a puppy (You work all day, right?) so off to a rescue or shelter you go.
There are several schools of thought on this. One says if you have a girl, get a boy, or if you have a boy, get a girl. That’s usually a safe bet, but it’s not your only option. With proper introductions and an understanding of how your dog thinks, you should able to add another dog to your household with minimal issues.
Let’s look at introducing the dogs to each other. If you are getting your dog from a rescue or shelter, chances are they will want you to bring your dog so the two of them can meet. This is a good idea because the dogs are meeting for the first time on neutral ground and (hopefully) supervised by knowledgeable people. If the dog you hope to add to the family is coming from a source other than a rescue or shelter,or they do not want you to bring your dog to meet the new guy, then you need to find a neutral place where they can meet. A park, or even a parking lot will do. No toys, food or anything else that could bring competition into the picture. Just two dogs, on leash. Watch the body language. Are both of them wagging their tails with their tongue’s hanging out, seemingly happy with the world? Good! Is one of them avoiding eye contact, while giving the other dog a sideways glance? Is one of them holding perfectly still and looking tense? Maybe not good. In either case it’s important to go slow and make sure the dogs are under your control. The meeting went well? Great! Let’s go home.
If your dog has been an only dog for his whole life, he is used to getting all his food, playing with all the toys, fetching the only ball…well, you get the idea. Adding another dogs brings something new to the family. Competition. Be very careful. Try to avoid a situation where both dogs are going for the same thing…at least for now. You’ve all heard the phrase “childproofing your house”? You should “puppy proof your house” for the first few days. Remove any toys or bones and other items that are laying around that might cause friction between anyone. When you feed the dogs, feed them in separate areas and do not let one go over to the other until the bowls are picked up.
When you walk them, try to walk them on either side of you. When you play with a ball, try not to let both dogs run to it at the same time. Remember, this is only temporary. In a few days, the two of them will become more comfortable with each other and slowly, things can return to normal.
Keep in mind your old dog has had the house, and you, all to himself and bringing a new dog into the house is a huge change. Take it slow and before you know it, they will be playing and snoozing together!