How can I filter related content types in Drupal?
You could populate the second list manually by using the PHP code option to generate the values, the values could be something such as ProjectNid|TaskNid|Task Title.

You could then have a javascript hook so when a project was selected from the dropdown, it reads the nid value and then filters out the tasks where the first element (Project nid) doesn't match.

The other way would be reload the page after the project has been selected, and have the PHP code which populates the task field look at the current active value for Project and pull in the related tasks, or use a view with an argument set to achieve the same result.