Final answer:
In aviation, the pilot and co-pilot, also known as the first officer, typically cannot be on crew rest at the same time to ensure that the flight deck is always attended by a sufficient number of qualified crew members.
Step-by-step explanation:
The question 'What two people can't be on crew rest at the same time?' pertains to aviation regulations, specifically those related to crew resource management in the cockpit of commercial aircraft. In the context of aviation, it is essential that there always be a sufficient number of qualified crew members awake and at their stations to ensure the safety of the flight. Therefore, the two people who typically can't be on crew rest simultaneously are the pilot and the co-pilot or first officer due to the necessity of always having a pilot in command at the controls and a second pilot available to assist with navigating, communicating, and other cockpit tasks.