What about when the margins are asymmetrical? Say, the top element wants 72px of space below, while the bottom element only needs 24px?
The bigger number wins.
This one feels intuitive if you think of margin as "personal space". In this moment in history, it's socially responsible to keep 6 feet apart. If someone wants even more space — say, 8 feet — we'll need to keep 8 feet apart in order to satisfy both personal-space requirements.