I know this is hard advice to follow but you have to let the dogs determine who is the alpha between them. Let them squabble (within reason). A show of aggression when a new dog is introduced is not really aggression at all. It is just dogs sorting out that pack heirarchy. It can be more difficult for the human than the dogs. Remember, they don't reason like humans...they reason like pack animals.

Which ever one is the alpha, needs your reinforcement of that status constantly. You need to do everything for the alpha before you do anything for the other one or you will see the little sqabbles get worse over time and they will never become friends. If you do your job of reinforcing the alpha, then harmony should rain in your home soon. Be patient.