I've just looked into this and can find (cant you tell I'm having a great Friday!) Ac/Inter (85000), Lazio/Roma (80000), Sampdoria/Genoa (37000), Chievo Verona/Hellas Verona (39000), Atalanta/Albinoffe (25000), Juve/Torino (27500...new ground being built but not sure if both teams are to play there).
All them stadiums are much bigger than anything we would build (sharing or not), and all them clubs are (historically) from the same town/city as each other. They have all pretty much played in a shared stadium for the majority of their history, and by playing in a shared groun they are not losing any of their identity (as its always been the case)...whereas if we shared with 'Dale then I think both clubs would be losing something.
Also....us and whoever we share with would have to rely on the other team being successful. If we shared with Rochdale, 50/50 split of the ground, and one of us went out of business, the other one would be knackered. In Germany, 1860 Munich almost went out of business and had to sell their share of the stadium to Bayern to survive, so now instead of being co-owners of a superb stadium, they are their rivals tenants and asset-less. Hardly ideal, and could easily happen to us.