I do know people who have made this work. But there are lots of factors to consider before you decide to do this. In general, for most families, getting two puppies from the same litter can be problematic.
1) Dogs are social animals. Young puppies form strong bonds with their siblings before they meet their human families. Bringing your puppy home with its litter mate interferes with the bonding process which must occur between a dog and its new human family. The presence of a littermate slows this process. In fact, most litter mates exhibit a strong preference for the company of each other over the company of their human owners.
2) People tend to neglect socialization with other dogs when they have 2 puppies. By the time the puppies are about six months old, the primary windows of socialization have closed. Two dogs who have grown up together and have been denied the experience of meeting lots of other friendly dogs of various ages, may only enjoy the company of one another. Some of them will become fearful or aggressive toward other dogs.
3) Owning a puppy is a huge responsibility. It requires a lot of hard work to raise even ONE puppy properly. Raising 2 puppies is twice the work. For starters, each puppy will require its own crate. Also, you will have to find time for 2 training schedules, along with separate times for training, exercise and play.
4) Housebreaking will take longer. There will always be one puppy who starts to "get it" before the other one - then that one will regress when he or she smells indoor elimination by its litter mate. This cycle of regression can continue (and even switch from one puppy to the other) resulting in much more time needed for housebreaking.
5) Learning each new obedience command requires a three phase process (instruction, correction, distraction). But when you have 2 puppies there is a built-in distraction (each other) during the first 2 phases of training. As a result, it may take longer for each puppy to learn basic obedience commands and household manners.
6) The less confident puppy will "hide in the shadow" of the more confident puppy. Therefore the less confident puppy may grow up lacking social skills and may exhibit a variety of fear and confidence issues.
7) Most breeders are dedicated and ethical. But there are some unscrupulous breeders who will tell a potential buyer that the last two members of the litter are "really attached to one another" and will lay a guilt trip on the buyer, sometimes even giving a discount to purchase both puppies. There is absolutely no merit in the idea that you're doing emotional harm to a young puppy by separating it from its sibling. (In fact, the opposite may be true.)
8) In order for proper social development to occur, a puppy must LEARN TO BE ALONE. This part of their training becomes more difficult when there's always a sibling nearby.
9) When the puppies reach adolescence, competition is intensified and fights can break out. Fights between litter mates are much more severe than fights between unrelated dogs.
10) In situations where the owner has decided to re-home one of the two puppies, each of the them began to form a much closer bond with its human owner, and made great strides in obedience training, confidence and sociability. This was most noticeable when the re-homing was done prior to 6 months of age, but I've seen it happen with older dogs and puppies as well.
There is no reason to bring home 2 puppies from the same litter and lots of reasons not to. But if you've already taken the plunge, follow the above tips (separate crates; separate time with each puppy for training and play; time for each puppy to meet and play with other dogs without the littermate present) in order to increase your odds of success. And be sure to provide appropriate control of resources and clear human leadership in the home to prevent fights as they get older.
One final thought: If you really have your heart set on owning two dogs from the same breeder, wait until the first dog reaches social maturity (about 18 months old for small and medium breeds, about 2 years old for large breeds, and 3 years old for giant breeds) then bring home a new 8 to 10 week old puppy (of a different sex). It will still be important to provide everything I just mentioned for the proper development of the new puppy, but by waiting until the first dog is an adult, you'll be more likely to avoid "litter mate syndrome."
