Ok, so you have a Brock's Golbat as your active, and you attach a PlusPower.  Does Spiral Dive 2nd attack do 20 to every opponent pokemon? 

PlusPower will only deal additional damage to the defending Pokemon.  Opponent's Benched Pokemon are not considered defending.  Golbat's Spiral Dive will, though, deal additional damage to the Opponent's Active Pokemon, though.

Scott