how to display common items in two list views in a third list view??

Question

I am making a chat messenger. In the messenger I am able to get all the people online in the server. Aclly there is a client side and a server side. And I have to link that chat messenger to a database where all his friend list is stored. So only the people who are both online and are in his friend list will be shown. I managed to get all online in the server in a listview and all present in the friendlist in another list view. Now to achieve my aim , I just have to select the usernames that is common to both the listviews and put them in another list views.

If any one has any alternate, less processor consuming idea of implementing this please inform.