There are scenarios where you'd like to delegate a task to more than one person e.g. get two team members working together on the same problem, XP pair programming, etc.

To avoid the obvious problem, the task is only marked complete for the delegator when both delegatees mark it complete (or otherwise leave it to the delegatees to decide and one of them marks it complete for both).

Perhaps these can be achieved by using a kanban card for the whole task but this doesn't always fit in with how the project admin wishes to scope his/her tasks and there will still be small-scoped tasks that will always require more than one person to complete (e.g. pair programming).