Ya, there's no "real" order of primacy in our diptychs. Our diptychs are also not fully established really. Apart from concelebration, our Coptic liturgies only commemorate Alexandria, then Antioch, then Ethiopia, then Eritrea (for some time, Ethiopia was excluded due to friction, which was brought to reconciliation by the Armenian Catholicos between the late patriarchs of Alexandria and Addis Ababa, Pope Shenouda and Abune Pavlos respectively).
But when there is concelebration or patriarchal meetings, the "host" gets the primary honor. Otherwise, it would seem to go Alexandria, Antioch, Armenia, then God knows what comes next.
I personally would like to see a sense of "term" honor primacy. I know in the ancient world, Rome, Constantinople, Alexandria had primary honor in the empire, but perhaps today, we should perhaps do something that is similar to the ancient Jewish laws. Just as St. Zacharias was chosen by lot to enter the temple, perhaps we should choose by lot who would convene the patriarchs and hold primary honor and arbiter-ship