They may not end up friends, but they can become tolerant. And it will always need to be supervised time together. I have introduced a few of my birds to each other. Jordan and Pickle I trust together totally now, pickle was our handraised baby, so that was rather easy.
It will all depend on your bird and how you integrate them when its time.
Its almost time for me to integrate my quakers, and having them in a separate room, which is in the second living area off the kitchen means, I can bring jordan and pickle out to guage their reaction. I did this a few days ago, they were not close to the quaker but could see them well and hear them. Jordan flew off like a maniac screaming, so I brought him back and again he did the same. Later that day I brought jordan out again. The next day I did the same with treats in my hand to distract him and he suddenly seemed to be inquisitive about the quaker, he started talking and responding to their sounds. So I sat down away from the cage but close enough they could see each other. I gave jordan a grape and we sat there a while. Well ... Billy our male quaker flew to the bottom of his cage and guess what he picked up, a grape he had dropped and wasnt interested in earlier and began to eat grape with us.
When it is time to introduce them properly I will move the cage to the entrance of the bird room for a few days and let them see and hear each other, its all a slow process, but well worth the effort.
Once they are integrated and all is settled, I will get them out for supervised time together.
I know billy and bell are a proven breeding pair, and some say I should leave them alone as a breeding pair, but billy is tame and I have had him out playing with me, and bell is getting tamer by the day also (she will never be really tame, but will be less frightened of us). My Indian Ringneck breeding pair are the same, Kai is tame and mickey is rather wild and they still produced beautiful babies for us last season, from their cage inside the bird room.
Anyway sorry for the essay. To sum up; integrate them slowly when its time, but do not ever leave them out together unsupervised.