Heh... there are a few instances where double weakness comes into play....
Armaldo-ex vs. Tyranitar-ex and Shiftry-ex (UF): Spiral Drain will do 160 damage if there is a React Energy attached to Armaldo-ex; Vortex Chop will do 400 damage.
Magcargo (UF) vs. Aggron-ex, Metagross-ex, Steelix-ex: If Dual Armor is in effect, Extra Flame will do 280 damage.
Blastoise d vs. Walrein-ex: Skull Bash does 240 damage, while Enraged Linear Attack can do as much as 400, or even 480 with Energy Root.
Some of these setups are quite contrived, and probably won't happen very often, if at all.