As I've mentioned before, over the past year, the greys have been working on their relationship. They appear to enjoy being near each other, calling back and forth, etc., without yet wanting any physical interaction. It's been fascinating watching this evolve and I imagine it will continue to evolve over the next few decades.
They keep a close eye on the other, get jealous from perceived injustices, and generally have a healthy mini-rivalry brewing.
Usually, they are content to sit on their stand (Max on the top level, Stella on the middle level) without problems. However, several times a day, they decide to irritate each other.
When Max instigates, she goes to the cup holder and lunges towards Stella. I couldn't get a picture of that as Max decided to pose for the camera instead! Mild beak-wrestling results and either one (or both) will fly off and land on different perches, or I separate them. I try not to get involved whenever possible as I'd like them to solve their own issues. If they were different sizes, couldn't fly away, or I thought someone could get hurt, then I would make sure they were kept separate -- solving their own problems is not worth risking an injury!
When Stella decides to irritate Max, she climbs up and makes noise while clicking the cup holder. As shown here, Max is usually oblivious. She rarely will go down to Stella, and Stella will not go any higher on the stand than she is here.
I usually see these kinds of confrontational behaviors from them, as you might expect, surrounding resources. If I'm giving Max head pets, Stella walks over to the cup holder and tries to get my attention focused on her. If Max finished whatever food I'd handed out and Stella still has hers, Max will go down to the cup holder. Occasionally, they appear to be unprovoked, but there certainly could be grey parrot messages being passed that I don't notice that compels these types of displays.