It’s informal.
In some cases, Icons are given as gifts by well meaning but not well informed friends and family. They might not realize St. Alexis Toth is considered a schismatic by the Catholic Church. Or that St. Peter the Aleut was sainted by the OCA & RO for having been martyred by Jesuits…
In other cases, it’s a personal devotion to a particular Orthodox saint for some reason. (In my case, St Herman of Alaska.) This usually is a patron of some particular area or endeavor. As long as they didn’t actively teach heresy, it’s generally quietly tolerated.
In still other cases, (none in the 20th century saints, tho,) the saint is on one or more of the Catholic Calendars, but may have been hostile to the Catholic Church. (Several from the 1000-1300 time frame come to mind, but their names don’t at the moment.) They may or may not be part of that person’s faith tradition, but they are still a Catholic saint by being on the calendar of any of the Catholic Churches.
As far as the new saints, few are recognized… even informally… tho common prayers for the joint martyrs of the Nazi and Soviet pogroms, both Catholic and Orthodox, have been held in various times and places.
A rare few might have been declared venerable by a Catholic Bishop. His flock may then publicly venerate that person; until Rome does, it’s purely a local thing.