I worked on a small project where it was just two people: one back-end developer, and one front-end developer. The project was not important enough to hire more people, but important enough that the manager complained about the "bus factor" and asked us to plan our vacations responsibly (i.e. not more than one week at a time).

Because both of us were interested in the other part of the work, we offered a solution that we could spend some time teaching each other. At the end, both of us would know both parts of the work, and be able to substitute for each other during the vacation.

"No, that would be a waste of time."