I don't know, I hear mixed things. They always say not to get littermates. These two Golden Retrievers I watch were littermates and one just passed away at 10 years old and the other doesn't know what to do with himself. It's so sad to see.
They said it was easy as puppies though as they kept each other entertained.
Then these 2 puppies I watch, 5 month old Bouviers are also never apart so when they get separated they FREAK out... I think they would be benefit from being separate sometimes personally BUT these dogs do not get nearly enough attention or training (owners are rich and always away) so for that I am glad they have each other at least while they are alone.
But I do think training etc should be done separately. I don't think sleeping together is a huge deal nor do I think getting at 11 weeks is either.