We only allow one assignee per task, because we believe a clear assignment ensures that the job gets done.
In our own experience, a task must often be solved by multiple assignees, but also via multiple steps.
We then note all the necessary steps as checklist items in a chronological order.
As soon the first assignee fulfilled his part he will re-assign the task to the next person, who is responsible for completing the next steps.
This way the assignment will always be clear as well as the current status of the task.
All relevant persons (watchers) can easily observe the progress like that.