Here is the complete list of stories in which they both appear. The first one is an Italian story from 1967, though that story didn't prevent the confusion between these two characters among Italians, a confusion which I have recently analyzed in another thread. Mario Gentilini's article which regards them as the same character is from 1968, and the misunderstanding lasted for at least two decades.
Some people say that in the continuity of a story that features one of those characters, the other doesn't exist. Because Glomgold is popular in American stories while Rockerduck almost exclusively appears in Italian comics. Yet Flinty also appears in many Italian stories and like others have mentioned, there are quite some works that feature both, and also, both were created by Barks.
To me, Glomgold is the second richest duck in the world and Rockerduck the third in the world, but the second in Duckburg. Some Northern European comics have Glomgold living in Duckburg too, but I doubt that and can gloss it over by him just having a second residence there, because the city is an important financial centre in the world.
Some people say that in the continuity of a story that features one of those characters, the other doesn't exist. Because Glomgold is popular in American stories while Rockerduck almost exclusively appears in Italian comics.
In a way, that's what happened in Italy for a long time: Glomgold had been totally ignored, either because creators were not aware of him or considered him the same character as RK, while Rockerduck was not only frequently used, but also referred to as the second richest duck, a title held by Glmogold in the countries that used him and followed Barks.
Anyway, I am not sure if it is correct to say that "Glomgold is popular in American stories": like it has been said in another thread, the myth of "comics from the USA prefer Glomgold" is actually kind of a projecting of modern standards to the past. Before Gladstone started publishing Disney comics in the mid-1980s, the character had only appeared in three stories (all of them created by Barks) , and even after that, he rarely appeared in an American comic set in the standard universe rather than in the DuckTales universe.