People always struggle or avoid doing pair programming, and people always think it means that the developers' time now costs twice as much, but this is a total lie. When I worked at a place that did 100% pair programming with rotation every day, a new developer could hit the ground flying . Sitting next to someone who knows the system and can answer your question immediately means you get up to speed millions of time…
On the "mentor" side of such pair programming, it is extremely useful to have to verbalize things that you may have gotten away with only fuzzy knowledge of previously. When the new hire asks a question that makes you say "oh wow that actually is quite broken!", you win as well!