In an airplane crash, every single person died, but two survived. That should simply be because they are not single. Single is a relationship status here. This is more of a riddle. In fact, it is a riddle. The approach used in answering a riddle is not the same in answering normal questions. The word 'single' in the statement is only a definition of relationship status, and not referring to the people collectively. So, if every single person died in the plane, then the two that did not die are in a relationship.
Probably they are a couple, or individually in different relationships. Another thing about riddles is that they usually have a single answer for them. Someone might answer that maybe they have access to parachutes, and they were able to escape with it. No matter how your answer might sound good and intellectual, if it is different from the generally accepted answer, you are still wrong.