Humans are not good at multitasking, despite what many will tell you. They end up doing a bunch of tasks poorly. We are not machines and therefore can not slice up the time in ways that will be the most productive. Computers are pretty good at doing this, but humans are not at all.
You may be able to get away with doing two items at the same time. But, remember, most tasks will require full attention, and it is impossible to do them simultaneously. Even computers have to emulate multitasking when the number of functions required is greater than the number of processors. However, they do everything in an entirely programmatic way. So, they know how to handle the emulated multitasking.
If you can use computers to do multiple tasks, that can help you get more done in less time. However, you still will need to manage the time for those tasks to ensure it has all the inputs required and take care of whatever is produced.
Another problem humans have with multitasking is that we can not help our minds wandering off when doing mundane tasks. We forget what we were doing. It often requires us to retrace our steps to figure out what we may have missed. It is certainly possible to develop a system where you write down where you left off, etc. But even here, the more tasks you try to do this with, the more challenging it will be to manage.
The situation gets complicated when managers expect you to get multiple tasks done, and they do not seem to take into account the amount of time needed for all the charges. If this is your situation, try to find ways to offload or delegate to others. You may be able to convince management to outsource aspects of your job to external resources. More companies are implementing this as it is pretty cost-effective. There are some challenges when doing this, as you will need to manage those resources and deal with company trade secrets, etc. But, if you can make this work, it can be a tremendous productivity booster.
Your focus will improve if you can keep your tasks to one item at a time. You may need to determine the critical paths of the tasks you are responsible for. When you show your manager that you get higher quality work complete by working on tasks one at a time, you should convince him or her this is a better approach.
Until next time, stay safe, and keep the faith.