- The problem can be that programmer A is stuck on a piece of code or debugging: this is usually part of a bigger task which A has as the top thing in their mind, but for some reason can't make progress. Programmer B brings a fresh pair of eyes to the table, but looking at it alone makes no sense, because their mental model will be up and running much faster with A's help.
- Onboarding. Programmer B is new to the project and programmer A gives an increasingly hands-on crash course of the project workflow and B's first task.
- The high-level parts of major refactorings. Neither A nor B will have the whole existing structure of the code completely in their heads, but between them they have a more complete picture and stand a much better chance of not messing it up.
None of these should take more than a few hours, or maybe a day max, or you probably will go insane, yes. It probably also depends on personality, but you should hopefully not have any homicidal tendencies after just a few hours.