Yeah, two cities in Sardinia might be overkill, those extra provinces are needed elsewhere. Still, I definately think that Sardinia and Corsica should be separate provinces. Not only for obvious gameplay-reasons, but also because it makes no sense that if you conquer Sardinia, you get a complementary island for free. Corsica is a province in its own right, and should stay that way.
The Romans conquered them separately, after all...