I have worked with a number of clients over the years that have purchased or adopted two littermates or even two puppies from different litters but gotten them at the same time and raised them as if they were littermates. While on occasion this works out ok, oftentimes its an extremely bumpy road for both the puppies and the humans, who often have no idea what they are getting themselves into. Bay Area trainer Jeff Stallings wrote a blog post many years ago now on the potential downsides of raising two puppies at once. It was so popular that Bark Magazine recently published the article, bringing it to my attention again. If you are considering two puppies, this article if some serious food for thought.
Here is the link to Jeff's Bark Magazine article titled "Don't Take Two Littermates."