Yes, it is whichever city expands to the tile first. So in the situation where one city already has claimed the tile, “ownership” is clear enough. My question is about tiles that get absorbed into your borders after both cities have been founded. For example, one of the two might already have a horse resource, so the new horse resource I really want assigned to the other city. How can I control that? What if the new resource is in the second ring of one, but the third ring of the other, can I still cause the tile to be assigned to the city which is further away?