We have a similar situation here, Goffins / Senegal. Both are flighted.
Like you I have visions of the birds ideal behavior and work toward that. But to be honest, I dont think I would ever be more than an arms reach from them when they are out together. We usually have them out one at a time, or on their separate perches. But you have to keep focused on them because as you say, they are quick.
I am begining to resign myself to keeping them separate, or under direct supervision. The risk of failure and subsequent injury to them is too great. This solution works at our home for now. I do have a couple of flight harnesses enroute. I have an idea that they may respond to the suits/leads as would a horse to being ground broken. It is just a thought, so I will let you know how that works if you are interested.
Dave
Oh, and welcome!
