Introducing two dogs must be done slowly and carefully. Even if you have done your research on choosing the right second dog, it might not be an instant match.
If the two dogs are not happily accepting each other at first, supervise them carefully. Sniffing, posing and barking, the dog will communicate in their own language. It's not advisable to let the dogs carry on with intense growling, snarling, and snapping, as an established dog may not be play-fighting to defend his (or HER) home turf. A female dog most likely will make it her full-time job to establish dominance over the newbie: bitches are not subtle and will threaten with both barrels (and fangs) to stay in charge. Both your present dog and the new dog need to be informed that you are the head of the household and that they're both subordinate to you.
When introducing a puppy to your adult, be especially carefully as a full-size adult inadvertently can hurt a puppy. While dogs can work out most things on their own, some dogs can be intolerant or simply mean when it comes to interacting with a new puppy. Be ready to step in and take control when you see things getting too heated or hostile.
When things go well, some dogs take to each other in a matter of minutes; but that's not always the case, for some dogs it can little take three or four months. You should separate the two dogs for periods so that both can have breaks from the other. Always feed them in separate places, ideally in different rooms. You will need to continue to spend time with your original dog while also making time to socialize and train the new one.
Until the new dog and adult are getting along, remove food bowls, treats and toys from their shared environment. Present toys and treats to them individually for the first month.
Always treat both the same in the company of the other. Make each dog sit for a treat and wait while the other receives it. You can either walk the two separately or have a family member or friend walk the new dog while you walk your "first born."
Remember that dogs learn by watching, so you can always reinforce your present dog's obedience training while the puppy or new dog watches from his crate.
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Thanks to Andrew DePrisco of DogChannel. com for these great ideas!