We have divided the 60 mishnayot of shas for a rebbis yahrzeit among talmidim on two continents. Let us say thirty on each. Since they all cannot attend one siyum can one make two. Although by the time the later one makes, there already has been a siyum on what they have learned by those attending the first siyum on another continent. Or since they have not attended can they make their own siyum. The question will really apply to times when a siyum allows you to eat like a firstborn on erev pesach. Or let us make the question a bit harder. It has been divided in such a way that no one has learned a whole mesechta. So unless one is allowed to make a siyum a second time they will not be able to make it even on one mesechta.