Carmen,
Hmmmm,
Babs and Jenna are littermates. I separated them at 18 months due to recognizing that Babsy would not realize her potential staying in the same area with Jenna.
Heidi was raised with Whitney and Tori, littermates. Tori went home to my brother at 9 weeks and was back at 15 weeks. Whitney died at age 4. Tori, last year at 8. They were together until they were about 15 months. I could keep two in the same kennel, not three -- two would gang up on the third. By 15 months all three were kept separately.
Milla and Ninja are littermates and they were separated at 15 months. Neither are rocket scientists. Both are fine as pets. Ninja would have to be an only. Milla could be in with anyone. Ninja is more of a fighter, and Milla more of a moderate. Milla is the one that could succeed at just about anything, and Ninja the one that, well, probably wouldn't make it as a service dog.
Dolly and Bear were littermates. Dolly went home at 8 weeks, and came back to me at six months. (Bad scenario) She was separated from Bear and Beansy, until Bear went home at 10 months and Beansy at 12 months. Bear came back to me at 15 months when his owners wanted to spend the summer camping. I got a great bitch back. She has been separated from her litter mate though, completely. Bear and Beansy were together for 10 months.
Nikki and Nina are littermates. They were two in March. They are still in with their dam. No problems.
Personally, I think it is hard to know whether its nature or nurture. Every litter is going to have pups with different personalities. They will not all be alphas. And not all of them will have the right stuff to be a seeing-eye dog. At eight weeks you can make an educated guess. And you may be right a percentage of the time. But maybe the folks that got two, got one that would've made it regardless and one that wouldn't. How can we be certain?
I mean if we were getting litters of 7, and all seven went to separate homes, and litter after litter seven puppies passed the test at 12 months or 18 months; and then we started having families raise two out of each litter and suddenly we were getting 3 out of 7 instead of 6 or 7 out of 7. I suppose we could say that.
ETA: I can and do separate bitches if there are any issues. Most pet-owners don't sign up for that, and would find it terribly difficult to do. I wouldn't suggest it, ever.