If one has a province with 2 sites that drop, say, enchantment costs, do they stack? If I have a Ench 50 and an Ench 20, do I get 50 or 70 off? Or are they multiplicative?
no. IIRC, only the highest applies. I once had the unreal fortune of finding a 30 and a 50 conj site in the same province (with Kail "I loves the summons" lasa no less!!). I still only got a 50% reduction in casting. Can you imagine 3 Apsaras for 1 pearl? Or 6 Gandharvas about 6!?!
The easiest way to verify which is true is to create a custom map with a province having multiple reduction sites in it with map commands, see mapedit.pdf in the doc folder. Then report back.
I had three 20 conjuration sites in a single province... no stacking. Quite sad, would only have been slightly better than ultimate gateway if it stacked linearly, anyway, and shouldn't I get properly rewarded when the starts align?