
Well I have, wait for it, FIVE sets of littermates here. Well in reality really four -because one set had the dog come back when adult, so they didn't grow up together.
The main point is to separate them as much as only possible. The more time they spend together, the more problems you get. You really need two different training classes to go to for instance -yes you can swap and let each go once a fortnight, but it isn't quite enough when you first start off.
The breed makes a big difference. I only have one single problem with my Papillon littermates -they gang up together and are little horrors when at home. Chase cats, chase bigger dogs etc. Other than that, they have been far easier to get independent than the others.
I honestly think though you will never, whatever you do, get littermates AS good as single pups -there will always be some problems all life. My oldest set is 9 ½. And it isn't twice the work, it's more like four times the work -and it really takes away the enjoyment of having a puppy. But it can be done when there is no other choice.