As I pointed out, a caucus doesn't necessarily have to focus on common agreement between two groups. It COULD focus on the differences between two groups.
After all, the points of agreement between Protestantism, generally speaking, and Orthodoxy amount to little more than a bit of shared Trinitarianism (even the Orthodox understanding of the papacy is far closer to the Catholic understanding than the general Protestant rejection of it). It'd be a short conversation. And the rationale for excluding other Trinitarians (e.g. - Catholics) would be little more than spite.
Thus, perhaps, a caucus thread could be established between Pentecostals who believe in UFOs and Pentecostals who don't. Or between Anglicans who accept the ordination of women and those who don't. Or between some Protestant group that generally believes that women should and can be ministers, and another Protestant group that doesn't.
And these two groups could have at it with each other, without interference from others.
I mean, heck, we Catholics don't believe that any Protestants have any sort of valid Holy Orders, so we should be indifferent to the question of whether women can be Protestant ministers. We don't have a dog in that fight.
Even with regard to the question of the Real Presence, since no Protestants whatsoever have the True Sacrament, as long as the caucus participants were careful not to directly disparage Catholicism, what would we care if the OPCers went after the Lutherans on the question?
The difficulty with a “Protestant/Orthodox (exclusive of Catholic) caucus” is that there is an identity between most of our theology and most of Orthodox theology. It isn't even that we agree with the Orthodox on most things, it's that we share nearly all the same theology. Both of our spiritual ancestors sat together at the Councils to affirm Christ's two natures, the Trinity, the Theotokos, the rejection of iconoclasm, etc.
It's not that there is an Orthodox theology of the two natures of Christ and a Catholic theology of them, and ours is like theirs. It's that there is a theology of the two natures of Christ of the Undivided Church, to which both Orthodox and Catholic are the legitimate heirs.
Thus, to attack the “Orthodox” teaching of this theology is to attack the Faith of the Undivided Church, in which the Catholic Church shares as well.
And to try to hold a caucus that would be meant to discuss differences between Protestants and Orthodox while excluding Catholics from the discussion would be a little difficult, since little of what the Orthodox believe is not what Catholics believe, too.
sitetest
That's not quite true with regards to certain Anglicans (and possibly some Lutherans).
Thus, to attack the “Orthodox” teaching of this theology is to attack the Faith of the Undivided Church, in which the Catholic Church shares as well.
My suspicion is that there is an ongoing attempt to drive a wedge between the Catholics and Orthodox on FR.
The major disagreements between the Catholics and Orthodox are over papal primacy (the Orthodox do not reject the pope as a bishop and are typically willing to acknowledge him as "first among equals," but they reject primacy) and the Filoque (an issue that Protestants typically agree with Catholics on and non-theologians typically don't understand the significance of).