I don't think it's just the police, stuff like transport would be a major issue if both NUFC and SAFC were to kick off at home on Saturday at 3pm since both clubs draw a large amount of support from outside the respective cities.
Is it like this everywhere in England, West Brom and Aston Villa wouldn't play home games on the same day right? Just hard to comprehend because it happens so much over here in the same city.
West Brom and Villa isn't really a big rivalry. Birmingham City and Aston Villa certainly would never have two home games on the same day, at least when Blues were in the premiership.