Humans can't multi-task! I still meet people who dictate to me that humans are capable of multi-tasking. It astonishes me that people still believe this even when considering how the brain works. Sure we can multi-task basic functions such as breathing and walking but when it comes to thought processes this is a different matter altogether. This is because our brain is massively interconnected. At any given neuron, the activation level is largely a product of the other connected neurons. As such if you are strongly thinking about one concept which creates certain patterns of activation in the brain, you automatically create patterns of activation in connected neurons. Try to think about two problems at once and you essentially chemically nuke the required activation patterns needed to be cognitively useful in either case.
There are lots of wonderful articles on the subject but one of my personal favourites is this from Leo Widrich
It covers multi-tasking in the brain along with some hacks and tips on how to work at it. About the only thing it doesn't cover is mindfulness training such as meditation. I meditate mostly daily and find the practice very useful for helping me keep my mind in focus. Im not what Mike Monday calls a "Focus Ninja" but i'm a lot better than I used to be.