Pressure raise the foes PP usage. so, if we use Blizzard for 1 PP, it will be 2 PP. How about if we battle against two pokemon with Pressure ability (example Entei and Ho-oh), is it 4 PP or still 2 PP?
The ability only activates when the attack successfully hits a pokemon, so only if you use a move like earthquake that has multiple targets will you lose more PP.