When you are faced with needing to report on issues or tasks you are assigned to and you are not the primary owner you can use the following.
- Create a new task or issue report
- In the filters section create a new filter
- Search for filter: Assignment Users ID
- Set it Equal to $$USER.ID
After this report has been run, you will be able to see all tasks or issues you are assigned to, regardless if you are the owner or an alternate assignee.