Shih Tzu puppies

hey are both gorgeous female dogs and very well behaved. She has been socialized with children since she was born and we also have 3 small children. Come to find out from a few professional trainers your not suppose to have two female dogs together in the house especially from the same litter. It can cause behaviour issues in the future over each one fighting for attention and for them to prove what dog is more dominant. We were told it is possible there is possibility they can fight to the point where they will hurt each other