I want to copy all tasks and email messages I've sent to a specific person to a separate folder. Can I apply rules to sent tasks?

When you create a rule to copy all items you've sent to a specific person, that rule operates on task and meeting requests, as well as email messages. However, because the rule copies the task request, not the original task, this approach might not be the solution you're looking for.

If you haven't already, consider using the Activities feature of Outlook contacts. Outlook collects and displays related tasks, messages, and other items for each contact on the Activities tab. To set up new activity groups, open the Contacts folder's Properties dialog box and click the Activities tab.