They are two separate posts, it’s just cross-posting won’t flood your instances “all” feed. They would still appear as a post in /c/London and one in /c/NewYork with separate comment threads
Is that a grouping the user makes?
In the Reddit apps that had multi-sub functionality then I believe they were user created but this is a brave new world and we don’t have to do it that way. People or instances or communities could create multi-communities and people could subscribe to them so only a central file would be updated. If it stopped being updated or was too broad or too specific, it could be forked. I wonder if that could even be rolled into a possible future wiki system as it need only be a text-based file listing the communities.