We have personal experience with litter mates and were told those horror stories e.g. they will never really bond with you, they will ignore you, etc, etc,
We never set out to get 2 puppies however, when we got to the breeder (?%$$??- read puppy mill), she had 2 left and both in real bad shape. They were 10 weeks old and sometimes really played roughly, and we felt that we had to protect the female. Well guess who the bully is now... yep Cleo. They each have their personality Buddy being just endearing and a cool little dude and Cleo being the little buffon. We even had to remove their collar and tags (Buddy nearly choked on it once) cause he would grab Cleo by the collar and shake her.
What Ownedbycats said in her post is exactly what happens with our 2. Buddy is extremely protective of his sister and they have bonded with us but whenever we are not sitting with them, Cleo just cuddles up to her brother. I have never heard either yelp in pain and sometimes their play can get a little rough. That is what Cleo is missing since Buddy has been diagnosed with diabetes and not as playful as he used to be.
Training was sometimes a bit of a challenge especially when Cleo had hip surgery and had to be crated a lot for a period of one year to prevent Buddy from playing and running after her. Then Buddy was sick for a while and although not crated regularly, he needed more attention but Cleo would go over and lick his ear regularly.
It is a joy to watch them play together, keep each other company and watch over each other. They are fun and just plain adorable.... of course, we are not biaised
They are now 9 1/2 years old and we never regretted taking 2 dogs.
Good luck