It's unfortunately not possible natively, and I don't know any extension that implements such a feature at this time.
You should be able to set a global moderator group on every tag scope, but a permission that requires having two different groups at the same time is not possible.
I have wanted to implement a sub-forum/group feature similar to Facebook for some time but never got to it. Ideally it would allow to promote members inside of a group to moderator or admin of that group. It's something I might release as a premium extension in the future.
For this particular use case an interesting solution could be for a community extension to add the ability to list individual users on the Permissions page instead of just groups. This should be doable with the Flarum extension API.