Can you send screenshots of what you are describing? I understand images so much better than text!
I’m a bit late to this discussion, but we do it by simply assigning certain tasks to two (or more projects).
We have individual projects but then assign the milestones to an overall project as well, for example by client. This means we can continue to manage our tasks at the individual project level, but whoever needs oversight gets to see the same milestone tasks in the overview.