This is such a complicated issue I can't cover it all in a single post.
I have bought birds to be companions to birds already in my flock.
The most recent case is buying a second green cheek amazon for my first Green cheek amazon. I have had the new bird for over a year and while they get along well with each other outside of a cage they don't do so well in the same cage. I fully believe things will smooth out eventualy but you have to be able to take the long view.
I had 2 reasons for getting a second bird.
First I believe Luna my first GCA spent some time in a local flock of parrots in the San Diego area. She would get super excited seeing other GCA on youtube I was sure she was missing her flock.
Second was the fact that I had had 3 Amazons for many years and I was really missing the playfulness that more that one bird can bring.
So to sum it up I think in special situations getting a second bird as a companion can somtimes work there, is no way to know in advance how it will work out.