In a plane crash somewhere, every single person died but two people survived.
How is this possible?
The 2 survivors were:
a couple, and the only two that were in a relationship. Every single (in the relationship sense) was killed.
In addition to the other answers, it's possible the two survivors were:
Siamese twins. Since their bodies are linked, they are one "person", but also not a "single person" since there are two of them.
Anyone who was married survived. "Every single person" can either be interpreted as "all of the people" or "every non-married person."