It can be done, but it needs some careful planning. Firstly, you need to be sure your dog will accept another dog, just because she got on well with the first, it doesn't mean she will do so with another. Your first dog would have been there first which she would have learnt to respect, but as she's now on her own, she may well have adjusted to being number one and not take at all kindly to something else moving in. Consider your dog's personality and character, is she placid and easy going or bossy and challenging? If your dog likes to rule and the new dog is of the same character that personality clash is not going to be a harmonious relationship. No one can tell for sure exactly how an 8 week old pup will turn out, but good breeders will know their pups and will be able to match you with the one they consider most suitable for you and your needs. My previous dog wasn't bothered by anything and I introduced my second without an issue. She on the other hand is a mummy's girl who hates sharing and would never accept another dog in her home, not even for a fleeting visit!
Once you have worked compatability out, if you do ahead with another dog here's some tips for you:
Existing dog and puppy should meet on neutral territory, not your house, car or garden as your current dog will view them as hers. Ask a friend if you can borrow their garden or a room for introductions, preferably somewhere your current dog has never been.
Most adult dogs will tolerate behaviour off puppies they wouldn't in another adult dog, so a puppy is significantly easier to introduce, as not only will it be accepted better, it won't be seen as a 'rival' by the adult dog.
Encourage good behaviour by giving your dog lots of cuddles and some treats when the puppy is around. This will help make your dog associate the pup with good things and should make having it around a very appealing prospect.
Always do everything with your current dog first. The puppy needs to learn to respect your dog, so always feed your current dog first, always give her attention first, when you walk them take her before taking the pup, same for playtime etc.
Feed both dogs separately, if possible in separate rooms. The sweetest natured dog can get nasty if another delves into it's dinner, don't allow the puppy to push your dog from her bowl or her bed. That is asking for animosity and resentment. Also, feeding in different rooms means neither dog will know what the other has had. Some dogs will go suddenly fussy if they see or smell the puppy gets something different, like small children thinking their missing out, which is unavoidable when one's on puppy mix and the other on adult food! Stop the problem from occurring by feeding them away from each other.
Make sure you walk your dog on its own without the puppy. Make a point of taking her out so she can enjoy your company without another 'tagging along'. Yes it's important to socialise the pup by walking them together, but it's equally important your current dog still gets your undivided time and attention as well
Don't let the puppy pester your dog. Because most dogs will tolerate puppies, they won't always stop them when their getting too much and just allow them to carry on and put up with it. Don't allow this, step in and remove the pup so that your dog can have some peace, and definitely don't let the pup trail your dog to her bed then start pestering her again. A dog's bed is it's safe place, somewhere it can retire to when it wants to be alone and undisturbed , if you let a puppy go to its bed it will likely become stressed and anxious thinking there's nowhere to get away from it.
The dogs will sort out their own hierarchy between them, and provided it doesn't get nasty, don't intervene. People have far more problems if they try to decide the order, mostly because they automatically put the older/bigger dog in the 'top spot' but it doesn't work like that. You could have a house with four Great Danes and a chihuahua and the chi could be the boss of the lot. The dogs will know better than you whose 'boss dog' and whose 'underdog', and if they've sorted it out for themselves, each will be happy and content because they've put themselves in that position.
Finally, unless your heart is set on a bitch, consider a dog. Although most adult dogs will get on with a puppy of either sex in the beginning, when adolescence kicks in at around 6-12 months it can be a different story entirely. A dog and a bitch will generally get on better at any age than two dogs or two bitches, even if they've been neutered, although it helps significantly!
Hope that helps you and hasn't put you off, because it can be done and very successfully, it just needs a bit of planning and thought as apart from the very odd exception you can't just plonk two animals of any sort together thinking they'll be instant friends anymore than you can with two people

. Good luck with your new puppy if you do get one and enjoy her/him as they don't stay small for long!

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