You cannot pick more than one assignee for a task.
When working on a task, the person assigned to it must understand what they are required to do and what they are responsible for. If several people are assigned to a task at once, they may do the same thing or, conversely, fail to complete an important part of the task.
If you want to assign more than one person for a task at a time, try to divide the task into several smaller parts that each person can understand. This will only increase the likelihood of the overall task being completed successfully!