I read in the Changelog for 0.9.42 that Grav now supports multiple collections. But there seems not yet any documentation how this is supposed to work. Any hints would be appreciated as this would be a useful feature.
On your page where you are defining the content items collections (ie blog list view) you can simply add another, for example:
content: items: @taxonomy.category: blog @self.modular: true
This will add all pages with taxonomy
category: blog and also the modular child pages to the collection.