Hub to Hub association in SharePoint online

Right now, the hub to hub association primarily provides a search scope. As far as I know, you can't use the hierarchy as a "roll up" for news (at least not yet) - but you could, of course, select the sites in the second hub as a source manually. You should be able to filter news using the same approach you can use to filter news in roll ups for any site - you need to be using a managed property that is used on all of the sites and the metadata needs to be applied to published pages and you need to make sure the search index has run - but since the associated hub is not yet providing a roll up scope, you need to select the sites manually.