Littermate syndrome doesn't exist Originally I was going to try and tread lightly on the whole littermate syndrome but since littermate syndrome in and of itself is mislabel to describe neophobia and general unsocialized dog behaviors that result from lack of training and socialization I figured I would just go ahead and say what I've found in my readings of more than a few articles.
Edit: it actually doesn't exist reading more in-depth articles in past few hours seems that this syndrome is fake and that it's one of those things where people say oh your dog has behavior issues so it has this syndrome that you need to rehome the dog for. Real professional trainers and behaviorists do not use the term nor is it even a real behavior syndrome at all. Having two sibling dogs will not cause them to develop fake littermate syndrome.
Edit: not saying the behaviors aren't a thing it but that it probably shouldn't be referred to as littermate syndrome because it can occur in any dog pairing not just dogs related to each other the article I have linked explains other factors that can cause any pair of dogs to act neophobic or aggressive due to things that the owner did not do when the dogs were young so it's not exactly a real thing so much as failure to do any sort of training/socialization. Crating them separately did not factor into it so it really doesn't matter if you crate two dogs separately when they are young what matters is exposure and socialization. There was also no mention of it contributing nor even helping to prevent the falsely named syndrome that doesn't actually exist because it's probably just term that was made up as cop-out to say I didn't socialize my dog enough so now it has this thing called littermate syndrome.
Edit: Also training forgot to say training, although it's a good idea to have dogs have their own sleeping spaces in general gives them the freedom to not be in eachothers space all the time but won't stop them from stealing each other's beds.
Not using this as support just thought you all might find the article interesting:
https://winter2019.iaabcjournal.org/littermate-syndrome/Edit: Also please do not come and say I'm a know it all because I did research on this looked through articles I'm not trying to disapprove that littermate syndrome isn't a thing only that the term is not real in and of itself but the behaviors associated with said fake syndrome are actually prevalent when dogs are not properly exposed trained or socialized in general making most of the so-called preventing of it actually moot when it's really just down to normal training socialization and desensitizing by exposing the pups or dogs to things. Nothing really scary about it but saying littermate syndrome makes it sound like every new dog owner needs to fear it when it's really just a matter of standard things you do when raising dog.
Edit: I am also aware that Cardinal you are listed among general training advice but I am imploring you to please do more extensive research rather than just blurting out littermate syndrome and saying to look it up. Also instead of calling it that why not just say this is the advice I can give you instead of blurting out something that might make a new dog owner scared or nervous about causing their pup to become neophobic and unsocialized cause they are playing with their sibling in the house. You may be well-intentioned but perhaps it might be a bit better to just not blurt out things that may cause people to get scared or nervous. This kind of fear mongers things and can even cause panic among dog or puppy owners in general.
Edit: Not attacking anyone just asking that they be a little more careful about how they word and that take a little more care in researching cause I am a bit alarmed to learn that such a fake behavior syndrome exists for what has nothing to do with whether two dogs come from the same litter or not and more to do with how the dog is raised. I'm not out to get anyone in this thread I am however here to make sure that people understand that littermate syndrome is not real. The behaviors that the dogs exhibit being used to describe the syndrome are but the syndrome itself is not.
Some other interesting articles this is blog maybe not the most credible so not being used as evidence but still an interesting read
https://fantasypups.weebly.com/blog/the-littermate-syndrome-mythAlso a video debunking littermate syndrome by a real expert Actually a pair of experts in animal behavior the videos only five minutes long but it's very educational. The experts are Certified Applied Animal Behaviorists Dr. Suzanne Hetts and Dr. Dan Estep Like these people went to school have Ph.D. in this field of study which makes them more qualified than all of us to determine whether it exists.
https://player.vimeo.com/video/203347340/Edit: also not trying to be mean or rude or anything but if you feel that because I have said any of this and have actually come up with sufficient information to educate people with and do not like it I have no problems with leaving this thread and refraining from posting any farther.
Edit: Separating the two pups causes a distress response suggesting that they should not be housed together or in separate rooms is very detrimental to a puppies health mentally. By telling people to house them in separate rooms and not allowing them access to each other you cause them to have a stress response. All the pups really need is to be trained separately and socialized with dogs and people outside of the home. They can still bond and will still listen to their owners if worked with separately and not trained together as pups tend to distract each other also it's harder to train two dogs at the same time so it's much easier to work with them individually but not allowing them access to each other or shutting them off from each other when they are siblings is detrimental to the two in first place. You're not trying to cause these animals to distress you're trying to raise them together not separate them.
Although it's probably money saving to use one crate while the pups are small and graduate to separate crates when they are older but should they be shut off from each other no that's where I disagree cause that again causes the stress response these pups are littermates from the same litter they can and will form bonds with their owners and will be just fine. Provided they get the training and socialization. I decided to look at more scholarly stuff to talk about this. If you still doubt littermate syndrome is fake perhaps ask yourself can these behaviors be applied to more than just puppies of the same litter? The answer is yes they can and if I wanted I could make the argument that adding a new dog to your family with an already existing dog will cause the same behavior issues as that of two puppies from same litter based on the littermate syndrome myth.
Sorry for the long amount of edits. I go really in depth when I do extensive research like this.
there are some who do, when they know that the new ower can work with and own two puppy of them, who learned enough of the new home and owner that they know that there should be no much trouble,... 
....yes most give only one to a new owner, as they are already enough to handle one puppy....real much work with one alone ^_^, but this has to do with how much dog knowledge and if they can take care of two not only one puppy... and how much time and people a family got... how big a garden and yard one has the new home needs to be big enough....and so on.... they are happy when they can give two puppy to the same good home, as they know where to find them more easy, dont need to run a check up on a seond possible owner, kow the person where the puppy go and the new home and so on.....(i would have gotten the male to my female, from same litter when he was not already spoken for at this time 
and im still sad that it was like this)
