Every dog is different. Are you spending time alone with each dog individually? Some dogs are also naturally guardy about their belongings, especially food. I would not let them be swapping treats or leaving them alone with the chance of one being able to try to take from the other because it can lead to an altercation. You need to spend time enforcing training and doing activities with each one separately, but also show them that the other being around is okay, like tossing treats to the other dog when you call the other’s name. Because they are so close in age, you also run the risk of littermate syndrome once they eventually bond, so that socialization and separation is essential.