it's called littermate syndrome. It can be done successfully, to the full benefit of the dogs, but it requires keeping them almost entirely separated until you are done - only short play periods a couple times a day.
All training, exercise, etc should be done separately until they are older. Then, you are going to be doing everything 3 times - once with each puppy separately and then practicing with both dogs in the same room (one confined and one free and working with you). A fourth time if the dogs have had a bit of experience working in the same room so that they learn to obey you and ignore each other with both dogs free.
What a "pet person" views as raising two dogs at the same time successfully is often very different than what a "dog person" would view as successful though.
Also, 2 or more dogs together will often react to chickens very differently than either dog does alone. So, even if you have an older dog that has never been a problem around livestock, that can change when you add another dog to the mix.