Apologies is this is already a feature, I cannot find any documentation.
I would like to be able to view multiple categories.
For example visiting https://meta.discourse.org/c/support+dev/ would show me the contents of support and dev combined.
This is not possible yet (and I also think that you cannot achieve that using search).
I achieved a quick n dirty solution by adding a parameter to the URL, which when read added a class to the body tag allowing categories to be hidden or shown by CSS. I then added a new top nav with links to the 3 or 4 different parameterised home pages showing the filtered views.
Far from perfect but it at least allowed me to visualise how such a set up would work if catgegory filtering was available in core.