Pair programming. I know of no better way to level up engineers. The more senior of the pair is also often learning too, especially if you rotate pairs so that the inexperienced one has recently picked something up from another senior.
And if anyone's concerned about a drop in capacity, I invite giving it an honest go and seeing for yourself. Whatever is thought to be lost is more than made up for in quality, reduction of miscommunication and building the wrong thing less often. Another big boost is the ability to maintain a 'flow context' despite interruptions, as well as much less 'down time' not moving in any direction. Some individual quiet thinking time does need to be reserved though.
And if anyone's concerned about a drop in capacity, I invite giving it an honest go and seeing for yourself. Whatever is thought to be lost is more than made up for in quality, reduction of miscommunication and building the wrong thing less often. Another big boost is the ability to maintain a 'flow context' despite interruptions, as well as much less 'down time' not moving in any direction. Some individual quiet thinking time does need to be reserved though.