I’ve been told by Roman Catholics that they would find no problem with venerating Orthodox saints who are not just not-Catholic, but who were Catholic apostates or who were heartily anti-Catholic. St. Mark of Ephesus, who opposed reunion at the “Council” of Florence, and St. Alexis Toth, who was an Eastern Catholic Priest who led hundreds out of Eastern Catholicism and into Orthodoxy, and St. Peter the Aleut, who was tortured to death by Catholic Missionaries in California and St. Peter the Roman who chose, post-Schism, to leave Roman Catholicism for what he felt was the truth of Orthodoxy were all in the list. So it does happen.
In Orthodoxy you’re allowed to privately venerate non-Orthodox saints, but I don’t know of any who do other than some with St. Francis (who some ‘ok’ because he apparently appeared in a vision to some Protestants once telling them to become Orthodox and that it was the true Church). Though this private veneration is allowed we appear to be more strict about allowing veneration of saints who taught or preached or acted contrary to the beliefs of Orthodoxy. I know that there are many Catholic saints who it is considered wildly inappropriate to venerate, such as Thomas Aquinas or (currently only Blessed) Cardinal Stepinac.
There are also many saints who are considered traditionally Roman, but who the Orthodox venerate because they are pre-Schism, such as St. Patrick of Ireland, St. Gregory Pope of Rome, or St. Augustine.