Can't get my head around multisite

I have two domains, one Australian and one USA based. I'm trying to make a site that uses the same database and the same nodes, save that I want to provide some nodes to the Aussie site and some to the USA site. Each have their own themes

I have a taxonomy set up called "Locale" with "Australia", "USA" and "All" as the terms underneath it. But I can't figure out how I filter all the nodes so that visiting the Aussie site uses only nodes marked with "Australia" or "ALL", etc

I've looked at multisite configs, but I can't see anywhere where nodes can be shared or any way to filter nodes accordingly before they are passed off to either the US or AU sites.