I had my first grey, a tag, at 8 1/2 weeks old. A girl. When she was 2 1/2, we adopted another girl, a cag, 2 months younger. Once the new bird came, they tolerated each other, but because she is much bigger, she is the boss, and is accepted as such. I have always given my first grey all of the firsts, such as my attention, food and such so she wouldn't feel put out. But when the cag comes near her, even to preen her, she says, Uh uh, no way! and she will go to the other side of the table or couch.
I was worried about fighting, but that never happened. They both accepted each other instantly, and things are great. I have been known to wear a grey on each shoulder as I go thru the house doing things. B)