For the first time ever I kept 2 littermates from my last litter, a male and female. The puppies are now almost 10 months old. Yet when I have them together, outside where they can run, they still behave as if they were 8 week old puppies, wrestling, mouthing and slamming into each other. They do not behave like this with the older dogs, only with each other, nor do my other dogs behave this way with them or each other. The female puppy seems to usually be the instigator with the rough play, when it gets too rough I have to separate them as I am afraid they will hurt each other. Never having kept two before, is this still normal behavior for littermates this age? Will it eventually stop? It would be nice to be able to have them outside together with the others without them trying to constantly smash into each other!

Separate them before one or both gets hurt. Been there, done that. Won't do it again. Let them play with an older dog. I separate littermates by 5-6 months of age, sometimes earlier if they are rougher.

We keep littermates often. It sort of depends on the two puppies, how they interact. If they are old enough to play with the adults - anywhere from 7-8 months, then if there is that sibling stuff going on - I like to send them out with a different group to break up the routine rough play. I don't have any problems with keeping littermates.ktnj

This is why we never sell 2 pups from the same litter to a pet family.

I have kept littermates before and had no issues.
I seperate them often for training, socializing. And I correct the behaviors when they happen if I think they are playing too rough. I don't allow them to play unsupervised if I think there is a hierarchy change going on.
I have also sold 2 puppies from 2 different litters, (same age) to a pet family before... They had the first puppy for 5 months and I sold the second one to them after they had gone to obedience training with the first and proved that they could handle another. When the 2nd went home they both took classes together and the family adores both of them..
It really depends on the individual puppies and families. I said I would never do it, but in this case it felt right and it was. I wish all pet families were like this one.