Productive currently does not support multiple assignees, but here's how you can work around that by using Subtasks that can be assigned to multiple people.

Within a task, simply add as many subtasks as a task would have assignees and assign people to those subtasks.
In the example below, instead of having 2 assignees, I have created 2 subtasks with each having a different assignee.
Those subtasks will show up in their Tasks overview so they can always keep track of their work.

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Also, note that you can leave the parent task's Assignee field blank. The only required fields on a task are its Title and Task list.

For smaller tasks, you can follow the same procedure and create To-do items instead of subtasks:

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