Hi @Donna - the issue with that approach is the tasks will not stay in sync when duplicated (change made on one task in the high-level board will not be reflected in the lower level board). Also updates cannot be mirrored between boards (maybe I am misunderstanding).
If someone is tagged in a private board that they are not subscribed they cannot be notified.
Another option would be to check out the vLookup app to see if that can solve your issues (paid app)?
If visibility is the main issue, the best way to make this work is via Item View Permissions, however this is only available on the Enterprise licensing plan. This way you can have a single board and define who can see what at the row level.
If you are ever interested in learning more of the benefits of Enterprise and to see if it is a good fit, feel free to reach out to me any time.
Thanks
Mark