I thnik, the simplest way is to define individual assigned tasks into the Task Node.
The Task Node will leave after all task are attended.
<task-node name="task-node1">
<task name="TaskForActor1">
<assignment actor-id="actor1"></assignment>
</task>
<task name="TaskForActor2">
<assignment actor-id="actor2"></assignment>
</task>
<task name="TaskForActor3">
<assignment actor-id="actor3"></assignment>
</task>
</task-node>