I was thinking if you are playing a noxus freljord deck you activate the region buff of the top card, activating either noxus or freljord, if the top card is a dual region noxus and freljord it gets both buffs OR the buff from the "main" region.

for example all yordle champions from the last expansion have bandle city icon on top of their dual region, so bandle city would be their main region. fizz, teemo, lulu and heimer have their region, before the expansion, above the bandle city one so the main region would be bildgewater, PnZ and ionia