Listview into Listview

I have a collection of users, pointing to a collection of task.

I want to do a report with a list of all the users, and under each user shows a list of task of each.

So I thought to use a Listiview into another Listview, but it only works half, I can not filter that only show the task of that specific user.

¿How can I do it?

thanks in advance

Octavio