So what you can do is search for all tasks assigned by X and sort your results by Assignee by clicking on the little icon at the right corner of your screen (see screenshot below):
This is not robust enough. I want to see all of my tasks and all of the “assigned by” folks in one view. This could happen in the “my tasks” view OR the filter could allow you to enter multiple users or just display all “assigned by” users at once. Right now the filter requires you to enter information by user instead of displaying all tasks and all assigned by information.