How can I relate Activity to a second object in Salesforce?

SFDC does not allow this currently. You cannot add lookup fields or junction objects to Tasks to enable what you want to do.

Workaround today include creating your own Task object and recreating Task functionality. This is how Milestones PM Application on AppExchange works.

Pros:

You can relate a Task to multiple Objects in the System. Gives you flexibility to cusotmize however you want within confines of Force.com. Formula Fields, Roll Up Summaries, etc.

Cons:

You lose ALL the special features of Activities like:

Outlook Integration Popups (Nag Boxes) for Task Reminders Activities History Rollups on Account Activity Archiving etc.