How exactly does it work? I've created a spell and set the archetype to Dual Value Modifier: the first AV is DestructionMod and the second is DestructionPowerMod, while the AV Weight is set to 2.000.

That means if the magnitude of the spell is 20, spells should cost 20% less to cast, but become 40% more powerful (according to the wiki, the second AV's value is the magnitude * AV Weight, and 20*2=40). However, that doesn't seem to be the case in-game, because both values are set to 20%.