[quote=viktor aleksndr]isnt it that during the requiem mass of HH JP2, there were members of the eastern church who concelebrated that mass.
Eastern RITES, yes. These Rites are under the full jurisdiction of the Pope. As Vatican II said,
These individual Churches, whether of the East or the West, although they differ somewhat among themselves in rite (to use the current phrase), that is, in liturgy, ecclesiastical discipline, and spiritual heritage, are, nevertheless, each as much as the others, entrusted to the pastoral government of the Roman Pontiff, the divinely appointed successor of St. Peter in primacy over the universal Church.
[right]Decree on the Catholic Churches of the Eastern Rite
[left]The Catholic Church does not concelebrate with members of, say, the Greek Orthodox Church, because (sadly) we are not IN COMMUNION with this Church.
For information about Eastern Rites, see this site.