Some collections questions

I’m figuring my way around 2.8, and have a few questions regarding collections.

  1. Is there some way to have a collection inside two different collections? Or is the collection structure strictly hierarchical?

  2. Is there some way to have per-viewport settings for display of collections, like unlinked layers in 2.79?

  3. What’s the quickest way to get the intersection of two collections? Like, to select all objects that exist in both collections.