Interesting question for a tax lawyer. If you think they qualify, would they be able to take the entire $500K exclusion on each sale? Or should they get only a combined $500K exclusion on the two sales?
Both may run into the problem that the exclusion is not allowed for any sale if the exclusion was taken on a sale within the previous two years. Of course, they could have made a single sale to a third party, who then sold the two units separately. Since they didn't, I don't know the answer.