Buying or adopting two puppies at once may double the fun, but it also doubles the trouble. Teaching one puppy where to potty, what to chew, how to greet people, and basic obedience overwhelms most owners. Training two at the same time often ends with one puppy re-homed.
Although puppies appreciate a full-time playmate, they require individual human attention daily to prevent them from bonding closer to each other than to you. The latter holds particularly true with already attached littermates, but easily happens when non-littermates spend every day together.
Even among littermates, fights frequently erupt upon maturity, especially in breeds that tend toward dog aggression and in same-sex dogs. Breeds whose original purpose included working closely with other dogs and opposite-sex dogs generally get along better long term. Sterilization prevents unwanted puppies.
Trainers rarely recommend adopting two puppies simultaneously, but if you have ample time, energy, and previous training experience, raising two puppies may prove an enjoyable challenge.