I don't believe that it makes any difference as to whether the mix is two females, two males or one of each, as long as one is not breeding, of course. No matter the sexual mix, I believe the relationships can be worked out, as long as the human has an understanding of the personality dynamics and a clear set of family flock guidelines. Further, there are many examples of households where Greys get along well with other parrot types too. It all depends on your personal situation. There will always be some levels of conflict when a new flock member is introduced, and therefore, it is wise to keep them physically separated in different cages and if possible, in different corners of the room, so that they feel they have their own territories.
Parrots can be pals and keep each other company, even living in separate cages.
Prepare your current flock. Observe the behavior of your flock and the new arrival. Observe your own behavior. Set up flock guidelines. You ARE the flock leader, and therefore, it is your job to set the tempo for the flock.