Aspiring pilot: Can meditation/yoga improve focus & multitasking under pressure? Might you have any other tips?

What an interesting question! Meditation can absolutely improve focus and the ability to multitask. One of the most important things we do when we meditate, is focus the mind and there are many ways in which we can do this. The first step I would suggest, is to consider breathwork practices, specifically box breathing and even alternate nostril breathing. Something as simple as focusing on breath, can have an incredibly fast, balancing effect on the nervous system, thus giving you the chance to make critical decisions in times of high levels of stress, using that logical, rather than emotional or instinctual part of the brain, which is very often unhelpful in high workload situations.
Is it possible to expand capacity? I suppose that would depend on how you would define capacity, but by focusing the mind, like we do in meditation, you will be able to quieten background noise-like thoughts and navigate around things that would normally present hurdles for you. I really believe that possessing the ability to calm the nervous system, to turn on that parasympathetic response, is a fundamental skill that pilots need to hone.