Dear Andreas:
In the Middle East, Catholics are allowed with permission to receive Holy Communion in The Orthodox Church where there is only that church in a village. The muslims won't allow many churches to be built, so Catholics and Orthodox have to share one building.
In some Orthodox churches in America, the Orthodox bishops also allow this, where there is no Eastern Catholic Church for the Melkite Greek Catholics to attend. It is limited, but in these situations, the Catholics must also regularly go to Holy Confession to the Orthodox Priest, observe the fasts and be repentant.