As I stated above, the effectiveness should MULTIPLY. If an attack deals double damage to 2 types of a corresponding dual-type, eg Brick Break against Bisharp (STE/DAR), the power is doubled twice (4x effective), whereas if an attack is super effective against one of a Pokémon's types but neutral to the other, eg Waterfall against Excadrill (GRO/STE), the attack is 2x effective, not 3x effective. 2x1=2, not 2+1=3. In the rare case of a triple weakness as used in these examples, the type effectiveness is 2x2x2 or 2*3, a neutral effectiveness of 1x is essentially doubled three times. In contrast, using an Ice type attack on Dewgong (WAT/ICE) with the ability Thick Fat is 1/8x effective. The math here is 0.5x0.5x0.5 or 1/2*3.