That’s easy enough, Paul. If a bishop in the Roman Catholic Church separates from his brother bishops who are in communion with the Pope, then that person has separated himself from the visible communion of the Roman Catholic Church, no? This really isn’t very different than in Orthodoxy. For instance, in my communion, there have been issues with the Ethiopian ‘Synod in Exile’ (formed by Ethiopian Orthodox Tewahedo in exile from communist Ethiopia during the days of the Derg/Mengistu Hailemariam regime, which heavily persecuted the Church), which separated itself due to the political situation in the Ethiopia of its day, and refused to recognize the Patriarch elected then, HH Abune Paulos (RIP). The rest of the communion, however, did recognize HH Abune Paulos as having been canonically elected, and hence those who continued to not do so thereby placed themselves outside of communion with the rest of the Church over this issue. There are perhaps shades of the ‘Western Schism’ and Avingon papacy in this (insofar as it concerns rival claimants to the same seat), but I don’t want to draw too close a parallel, only to point out that just as that period ultimately did not destroy the Roman Catholic Church, this likewise does not destroy the Orthodox Church, as it is not as though there is no way to know who is following the rules and who is not. (Rather, that is many Roman Catholics’ idea of what Orthodoxy must be like since we “don’t have a Pope” – except we totally do, HH Pope Tawadros II – so therefore everything is
craaaazy and nobody can ever really know up from down or whatever.

)
By contrast, the opposite situation exists in modern Eritrea, where the government of Isaias Afwerki illegally removed the Eritrean Patriarch, HH Abune Antonios, and replaced him with a subservient puppet elected by a government-gathered (and hand-picked) ‘synod’, who placed an imposter patriarch by the name of Dioskoros on the throne and threw the actual Patriarch in jail (he’s been in secret detention for years now). This is not acceptable, and the other Oriental Orthodox churches do not recognize his deposition, much less his replacement (the people of Eritrea, I’d imagine, do not have much of a choice in the matter, sadly, but there are still those who recognize HH Abune Antonios, as in the majority of the diaspora, and the Christians who have bravely defied the government in-country, including many monastics). Here the diptychs come in handy again, as you can tell by them who is a part of the Church and who isn’t. In the Coptic Orthodox Church diptych, HH Abune Antonios is
always mentioned, both by preexisting agreement and custom (the churches of Ethiopia and now the independent Eritrea being the daughter churches of the Coptic), and to underline exactly where it is we stand on this issue.
You might then be thinking to yourself “Aha! What a mess! They wouldn’t have this problem if they had a supreme, infallible bishop as we Catholics do!” Hmmm…well, disregarding the fact that you already have had that problem in the past despite having a bishop whom you claim is supreme and infallible (see: the aforementioned Western Schism), there is also something to be said in the above examples against the unfair and wrong characterization of the Orthodox as “stagnant” and unable to respond to modern problems, for some reason. I should think that these two examples (which are just two that are well-known in the OO communion; the EO have had their own issues in the recent and distant past, but it’s not my place to comment on them either way, since that’s not my church; I do notice that they deal with them much as we deal with ours, though) speak truth to those lies in that nobody can say that we are without problems, but also nobody can say that we are without solutions to said problems, at least as much as any Catholic will say is provided by their infallible bishop, Pope Francis. After all, what does the RCC do about its various splinter groups such as the Old Catholics, the Evangelical Catholic movement (
these people, I mean), not to mention the various Protestant churches
not claiming the name ‘Catholic’? You say “We don’t recognize these people, no matter what they name themselves or claim their pedigree is”, right? And you don’t commune their members, and you don’t allow your members to commune with them, right? Welllll…that’s exactly what we do about everyone outside of our communion. So it’s not different. Catholics may say “Well, sure, but where does your AUTHORITY to do that come from, without an infallible bishop?” This is a non-question, as our bishops do not need to be infallible in the first place to see who is and who isn’t acting according to the norms of our communion…just obedient to the Fathers and the canons and laws as govern their church, same as any non-infallible Catholic bishop (i.e. bishops who aren’t the Pope) would have to be.
So, I’m sorry, but I think that the RCC position gives its faithful the idea that they have so much more assurance and unity and clarity and this and that than the Orthodox must have, but in practice there is very little to point to (save the Roman-specific interpretation of this or that verse of St. Matthew’s gospel, which…ehhh…as Protestants have shown us, merely having an interpretation of the Bible is no guarantee of anything) that actually bears that out. In practice, they are two different models of how the Church should be run and authority recognized and wielded, but given how they’re actually used, it’s really silly for Catholics to continue to insist that they have all this magical unity glue in their Pope that somehow we don’t have just because we don’t treat any one person as though they are infallible and the locus of unity around all must gather in order to be considered Orthodox.