* Milan -> Lombardy
* Turin -> Piedmont
I'm normally not much involved in the discussions re hotspots. Just would like to point out that this regional approach makes, in my opinion, little sense for a traveller. If you're stuck in a little Lombard village, you won't necessarily be able to do a day trip (at least per public transport) to more out-of-the-way places at the other end of Lombardy.
On the other hand, if you're in Milan, it is perfectly possible to do Turin, Verona, etc per high-speed train. Did Turin a few years again, it was a breeze. For travellers, knowing that you can do Turin as a day-trip from Milan is probably more useful than knowing which sites are located in Lombardy. If you start doing "Lombardy", "Piemont", etc. then we'll soon end up with "Sites per sub-national divisions" - with "Sites in Hesse", "Sites in Wales", "Sites in the State of New York" next.
My suggestion in case of overlapping hotspots would be to choose the city with the biggest network of transportation as the centre of the hotspot. I.e. Milan, Frankfurt etc. These are the cities most travellers would likely choose as a base for exploring surroundings.