Steps to reproduce:
- Add a task named Task 1 as a task in Project A.
- Add a task named Task 2 as a task in Project B.
- In the detail pane for Task 1, select *Convert to Subtask Select Task 2 from the dropdown list.
That Task 1 becomes a subtask of Task 2, without joining that project, but it also retains its association with Project A.
This solves the workload and portfolio issues because you can easily choose to put or not put both projects in the portfolio and get the full picture of the workload.