There is one significant factor to consider for imagining the diptychs of a united church: is this taken to be a bilateral union, or one party rejoining the other that is the Church?
I'm taking it to be a bilateral reunion... And in this situation, I'm kind of imagining that the reunion would take a while and isn't the kind we are necessarily thinking of. I think reunion will come with a gradual seaming back together through years of inter-cooperation, dialogue, theological discussions, uniformity in faith... This would be followed by inter-communion and con-celebrations and then eventually a formal declaration of a reunion that is already in-place.
(I'm thinking of it in a similar fashion to the way Saints are recognized)
In such a case, I believe the Bishops should be a bit reserved at first, and should be in contact with one another so they can be aware of diversions in faith. If no such divisions exist, I don't believe reunion would come immediately, at least formally.
To me, I think of it in a similar manner to the way someone is received into the Church. At a point in time, one may be a different faith entirely. But gradually they come closer and closer to Orthodoxy. They are "stiched" into the Church, and are formally received with the sacraments of Baptism and/or Chrismation and/or Confession.
Baptism (in my opinion) is an absolute official declaration. But there are some Churches that practice proper, canonical baptism, so a second baptism isn't needed. So Chrismation/Confession would be used to receive the person.
I also believe that there are some who are "invisibly" members of the Church, and won't be revealed until the end. Some of these people may not be baptized Orthodox Christians. But somehow, through the grace of the Holy Spirit, they've been weaved into his Holy Church.
I think this is currently happening with the EO & OO... While I believe the EO Church to be the one true Church, I believe that the tear between us is being gradually sewn back together by the Holy Spirit. That being the case, I think reunion will come before an official declaration.
I also don't think this is unheard of. In fact, this is the way the EO/OO schism occurred and the Rome/EO schism occurred. You had a gradual ripping apart, and then a formal recognition (or several) of a schism that was already present.
So too, we will have a gradual weaving together, and then a formal recognition of a reunion that has already occurred. (by the grace of the Holy Spirit)
But again, this is my personal opinion and how I'm looking at these things.